Background and aims: Obesity is one of the most important problems of modern society, and body mass index (BMI) is a tool to indicate overweight and obesity in adults. Obesity is directly associated with many non-communicable diseases. So for this reason, it was designed a study to assess overweight and obesity in Kooy dormitory students of Tehran University of Medical Sciences in 2015. Methods: A cross-sectional study was conducted on 342 students (210 girls and 132 boys) selected by random cluster sampling. Students' height and weight were measured and other data were collected by questionnaire and finally analyzed using SPSS. Results: The average age of girls and boys was 23.27±3.09 and 22.58±5.08, respectively. Findings showed that girls’ average BMI was 21.40±3.32 and boys’ one was 23.6±2.82, with significant difference between sexes (P