There is an obesity epidemic worldwide, which has been increasing in recent years. An epidemiologic cross-sectional study was conducted among 3799 persons who were 30–70 years old in Semnan Province, Iran. Multistage cluster sampling was performed, and subjects were selected from urban and rural populations. Body weight, height and waist circumference (WC) were measured, and body mass index (BMI) and waist to hip ratio (WHR) were calculated. Overweight and obesity were defined as 25 ≤ BMI < 30 and ≥30 respectively in men with WHR ≥ 0.9 or WC ≥ 102 cm, and women with WHR ≥ 0.8 or WC ≥ 88 cm were considered centrally obesite. Prevalence of obesity and overweight was 26.3% and 40.6% respectively. Prevalence of obesity was more among women (39.5%) than men (14.5%) with central obesity prevalence using WHR and WC cut-points of 72.2% and 26.6% respectively. There was a significant association between obesity and age, gender, residential area and educational level (P < 0.01). In conclusion, prevalence of obesity and overweight among 30–70 years old, especially among women, was higher than expected. A comprehensive educational programme on obesity risk factors and obesity-related diseases is necessary.