Background: Pneumonia is one of the leading causes of morbidity and mortality in children around the world. Annually, it’s estimated about total of 120 million cases of pneumonia occur in children under the age of five around the world and about 2 million of them end in mortality. In 2017, respiratory diseases constituted 31.3% of all diseases in children under the age of five in Ulaanbaatar city. 268 cases of respiratory diseases occur in every 1000 children, making it the leading disease in children of that age group. Sonographic examination has several advantages including lack of radiation, accessibility, portable (can easily bring it next to the patient’s bed), cost effective, and can be used repeatedly on the patients. To our knowledge, currently in Mongolia, there are no published research materials on sonographic examination in children with pneumonia. Given, the lack of study, increased rate of pneumonia and advantages of ultrasonographic, the present study was performed to detect lung ultrasound sign of pneumonia and determine the specificity and sensitivity of lung ultrasound and X-ray. Methods: The study used cross-sectional studies of analytical study. 379 inpatient children aged between 1 month to 14 years old who are in the children’s hospital of Bayangol district between 1st of September 2019 to 1st of December 2019 participated in the study. Procedures included collection of questionnaires from the parents or the guardians, random collection of data from the patients’ history, and assessment of results from ultrasound and X-ray examination. The study used Stata software to analyze the statistics. Results: 56% or 210 cases showed decreased echogenicity in the ultrasound examination. Incorporated B line sign (р=0.001),, decreased echogenicity in the consolidation (р=0.001), and presence air positive bronchus (р=0.001), are shown to the major sign of pneumonia in ultrasound examination. Conclusions In pneumonia, following signs are present in lung ultrasound: Unilateral B-lines, consolidation, hypoechoic, irregular shaped consolidation, positive air bronchogramm sign in lung. From the results of our research, we discovered diagnosis sensitivity of pneumonia in ultrasound is 85%, specificity of pneumonia in ultrasound is 87%, sensitivity of pneumonia in Х ray is 78%, specificity of pneumonia in Х ray is 84%.