AIMS: To evaluate the prevalence of associated risk factors of over active bladder (OAB) in people >/=40 years old in mainland of China. METHODS: A randomized community-based, cross-sectional study was performed on 10,160 residents (>/=40 years old) in the mainland of China using a questionnaire. The overactive bladder symptom score (OABSS) was filled upon site. The chi-square test was used to determine the differences of prevalence between sex, age groups, body mass index (BMI) and people with and without diabetes mellitus (DM). RESULTS: A total of 9,805 (96.5%) subjects qualified for the final statistical analysis. The overall prevalence of OAB was 2.1% (209/9,805), of whom 1.0% had OABdry and 1.1% had OABwet . The prevalence of OAB was more common in men than in women over the age of 60 (4.6% vs. 2.6%, P /= 25 (P /= 25 and people suffering from obesity are more likely to have OAB.