Background: Hepatitis virus infections are widespread and highly endemic in Mongolia and ranks first in the world for liver cancer mortality per 100,000 population, eight times the world average. The World Health Organization estimates that more than 2 billion people are infected with the hepatitis B virus. Each year, 1 million people die from the infection, 4 million are newly infected, and approximately 350-400 million are chronic carriers. In 2018, 475 cases of viral hepatitis were recorded nationwide, accounting for 1.1 percent of all communicable diseases, a decrease of 59 cases or 0.2 per 10,000 population compared to the previous year. In 2016, 194 WHO member countries joined forces to develop a strategy to reduce viral hepatitis, with the goal of reducing mortality by 65% and new infections by 90% by 2030. In order to achieve this goal, the strategic goal states that each country must conduct a comprehensive public health study and intervention on the spread of infection, risk factors, and early detection. Aim: Study to the coverage of hepatitis B immunization among children aged 0-9 years in urban and rural Mongolia and to determine the influencing factors. Materials and Methods: A Nationwide population based cross-sectional study design was used in this study. Mongolia is geographically divided into the western, Khangai, eastern, and central regions. A total of 14 provinces were selected randomly in addition to Ulaanbaatar city. The appropriate sample size was estimated at 4500 children aged 0-9 years, based on 2019 demographic data from the National Statistics Office. The questionnaire contained closed and semi-closed questions on demographics, socio-economic status, vaccination history and etc. Results: A total of 5027 children aged 0-9 years were enrolled in this study out of which 33.7% (n=1692) and 66.3% (n=3335) were enrolled from capital city Ulaanbaatar and provinces, respectively. Almost half (n=2552) of the study participants were boys whereas the remaining were girls 50.0% (n=2554). According to the history of Hepatitis B vaccination by questionnaire of parents’, 91.2% [91.2-92.0] were vaccinated with Hepatitis B. The proportion was 89.7% [89.7-90.8] and 94.1% [94.1-95.2] in rural and urban areas, respectively. Nearly 90% [89.6-90.5] of children were vaccinated in hospitals, 2.3% [2.0-5.0] were vaccinated at home, 8.1% [7.9-10.7] were unaware of the study participants’ location of vaccination. There were no statistically significant differences by urban and rural residences. Vaccination coverage against Hepatitis B was 91.5% (n=2300) and 90.9% (n=2284) among boys and girls, respectively and 89.6% (n=4506) were vaccinated at hospitals. Vaccination coverage were similar by sex. We also used Health Documents /pink book of children/ or vaccination card for each child to determine the coverage. According to the data from the children’s vaccination card and health documents’, 917 (18.2%) children were not vaccinated against hepatitis, 57 (1.1%) children received 1 dose, 235 (4.7%) children received 2 doses, and 3818 (75.9%) children received all 3 doses of hepatitis B vaccine. There was no significant difference by sex, though the coverage varied by age. For instance, proportion of children with no written documentation in the vaccination card was 13.5% among children aged 1 years that increased to 22.5% and 25.3% among children aged 8 and 9 years, respectively. In contrast the coverage rate of 3 doses of hepatitis B vaccination declined from 77.8% to 70.7% among children aged 1 years and 9 years, respectively. Hepatitis B vaccination coverage according to the vaccination card was different by provinces and within the districts of Ulaanbaatar city. Conclusion A total of 5027 children aged 0-9 years were included in the study, of which 917 (18.2%) children were not vaccinated against hepatitis, 57 (1.1%) children were vaccinated against the first dose, 235 (4.7%) children were vaccinated against the second dose, and 3818 (75.9%) children were fully vaccinated against the first-third dose. Although the coverage of the study participants varied depending on age and place of residence, no significant differences were observed in terms of gender. The current rate of children who are not fully vaccinated stands at 18.2%, emphasizing the need to ensure all children receive full vaccinations for hepatitis B and the required five doses as per the schedule. Furthermore, it is essential to mandate booster vaccinations for those with delayed immunizations and improve the accuracy of registration data.