Objective: Puberty is a critical time between childhood and adulthood. Many studies have reported that the mean age of breast development is decreasing. The aim of this study was to provide updated data on the pubertal development of girls and to evaluate precocious puberty in our population.Methods: This cross sectional study was conducted in 6 to 16 year old school girls during 2009-2010 in Qazvin. 2240 healthy girls from all geographical regions with every socioeconomic status were selected by a stratified multistage cluster design to obtain representative sample of population. A questionnaire includingdemographic data, anthropometric measurements, secondary sexual characteristics, menarche status and its onset was filled out for every participant. Secondary sexual characteristics including breast development(B1–5) and pubic hair (PH1–5) were evaluated according to Marshal and Tanner recommendation.Findings: The mean±SD of height, weight, and BMI of participants was 139.7±14.5, 36.1±12.9 and 17.9±3.7 respectively. The mean age (10th – 90th percentile) of B2 and PH2 were 9.71(7.67–11.4) and 9.82 years (7.84–11.42) respectively. Mean age of menstruation was 12.52 years. The mean BMI was significantly higher in pubertal females comparing to prepubertal girls (P<0.001). Average duration of puberty (the time from initiation of puberty to menarche) was 2.81 years.Conclusion: The mean age of pubertal onset in girls living in Qazvin is 9.71 years. Menarche occurs at mean age of 12.52 and onset of puberty earlier than 6.24 years will be precocious. We found that girls in Qazvin had a slightly earlier age of initiation of puberty and of menarche in comparison with other studies in Iran.