Introduction: According to the report of World Health Organization and International Society of Hypertension, an estimated 17.5 million people died from cardiovascular diseases in 2005 representing 30% of all global deaths, 1.5 billion people had hypertension and 45% of hypertensive people were unaware of their condition in 2007. In our country, prevalence of hypertension and its risk factors are studied, but prevalence of uncontrolled hypertension that is an important criterion of hypertension control, and its influencing factors is not reported yet. Goal: The aim of our study is to evaluate prevalence of uncontrolled hypertension and to determine its some influencing factors.Materials and Methods: It was population based, cross sectional, prospective study. Participants of the study were selected by randomized method. In the study, 1111 individuals that live in Bayanzurkh and Songinokhairkhan districts of Ulaanbaatar city are participated. The study based on questionnaire and BP measurement which aimed to reveal:• Hypertension unawareness• Untreated• Treated and uncontrolledStatistical analysis was performed by using SPSS 17.0 program.Results: In the study, in total 1111 people aged ≥18 years have been involved. Mean age of all participant was 40.58±16.10 and 63.2% of them were female. We estimated that 38.4% of the population had hypertension and 28.3% of those were unaware their hypertension (unawareness). Among aware population, untreated population percent were 42.1%. We determined that age, sex, education level and visiting to physicians can be risk factors of uncontrolled hypertension.Conclusion: Of the estimated 1111 people with hypertension 28.3% were unaware of their hypertension, 29.5% were aware of their condition but were not being treated, 33.7% were being treated but their hypertension remained uncontrolled and only 8.4% were taking medications that controlled their hypertension. Factors such as young age, male gender, and not measuring BP for the last one year influence negatively on uncontrolled hypertension (p<0.005).