OBJECTIVE: To study the association between body mass index (BMI) and the health-related quality of life (HRQOL) in the middle-aged and older Chinese people. METHODS: Data of 9539 middle-aged and older adults was collected from a cross-sectional survey performed in 9 provinces of China (Jiangsu, Anhui, Gansu, Qinghai, Fujian, Beijing, Jilin, Jiangxi and Henan province). MOS SF-36 was used to measure HRQOL. BMI classification was in accordance with the criteria recommended by the Ministry of Health of China. Rank sum test was used to compare HRQOL between subjects with normal weight and those with different BMI classification. Multiple logistic regression analysis was used to assess the association of HRQOL with BMI after adjusted for sex, age, marital, education, physical activity status and chronic diseases. RESULTS: When compared with middle-aged and older adults at normal weight range (18.5< or =BMI<24), data on physical domain (P<0.001), mental domain (P< 0.01) and 8 dimensions of HRQOL (physical functioning, mental health, P<0.05; role-physical, bodily pain, general health, vitality, social functioning, role-emotional, P<0.01) among subjects with underweight (BMI<18.5) were significantly lower while mental component summary (P<0.05) of overweight subjects (24< or =BMI<28) was significantly higher. Obese subjects (BMI> or =28) had worse physical function (physical functioning, P<0.01) but better mental health (mental health, P<0.01; mental component summary, P<0.05). After adjusting for other factors, and compared to middle aged and older adults with normal weight, data on odds ratios (ORs) of impaired HRQOL in physical domain (OR=1.67, 95% CI: 1.35-2.06), mental domain (OR=1.39, 95% CI: 1.13-1.70) and 8 dimensions increased among underweight subjects while ORs of impaired HRQOL in mental domain (OR=0.86, 95% CI: 0.78-0.95) and role-physical, vitality, social functioning, role-emotional and mental health dimensions decreased among overweight subjects. ORs increased (OR= 1.51, 95% CI: 1.27-1.80) in impaired HRQOL in physical functioning dimension but decreased in mental domain (OR=0.71, 95% CI: 0.60-0.85) as well as vitality, role-emotional and mental health dimensions among obese subjects. CONCLUSION: HRQOL of each domain were different among middle aged and older adults with different BMI classification. Underweight people had poor HRQOL in both physical domain and psychological domain, and obese people had poor physical function but good mental health condition.