We observed the effect of body mass index (BMI) on pregnancy outcomes in Chinese patients undergoing assisted reproductive treatment (ART). All the patients were divided into polycystic ovary syndrome (PCOS) group and non-PCOS group, and then according to BMI, each group was subdivided into 6 subgroups: group 1 (BMI < 18 kg/m2), group 2 (18-20 kg/m2), group 3 (20-22 kg/m2), group 4 (22-24 kg/m2), group 5 (24-26 kg/m2) and group 6 (BMI > 26.0 kg/m2). We found that in 20 to 25-year-old patients, the pregnancy rate was not significantly correlated with BMI in PCOS patients; while in non-POCS patients, the pregnancy rate significantly decreased at the BMI cut-off point value of 24-26 kg/m2. The pregnancy rate significantly declined at the BMI cut-off point values of 22-24 kg/m2 and 18-20 kg/m2, respectively in 25 to 35-year-old and in over 35-year-old PCOS patients; while in over 25-year-old non-PCOS patients, no significant correlation between pregnancy rate and BMI was observed. We conclude that for under 25-year-old non-PCOS patients, ART should be performed after BMI is controlled under 26 kg/m2. For PCOS patients, if age is 25 to 35 years or over 35 years, BMI should be controlled below 24 kg/m2 or below 20 kg/m2, respectively.