OBJECTIVE To explore the influence of body fat percentage (BF%) and body mass index (BMI) on the semen quality of adult males. METHODS A total of 125 randomly selected male infertility patients underwent examinations of semen quality, BMI and BF% on the day of enrollment. With BMI ≥28 kg/m2 as the criterion of obesity, 50 of the patients fell into the category of obesity and 75 into that of non-obesity, while with BF% >25% as the criterion, 69 belonged to the obesity and 56 to the non-obesity type. We compared the semen parameters of the subjects between the obesity and non-obesity groups based on the two criteria and analyzed the correlation of semen quality with age, BF% and BMI. RESULTS With BF% as the criterion, the obesity patients, as compared with the non-obesity men, showed significantly lower semen volume (［2.94 ± 1.15］ vs ［3.51 ± 1.27］ ml, P < 0.05), percentage of grade a+b sperm (［33.37 ± 19.80］% vs ［41.87 ± 15.43］%, P < 0.05) and sperm motility (［56.31 ± 22.26］% vs ［64.95 ± 18.22］%, P < 0.05). Similar results were observed with BMI as the criterion in the semen volume (［2.86 ± 1.11］ vs ［3.34 ± 1.26］ ml, P < 0.05), percentage of grade a sperm (［16.33 ± 13.80］% vs ［25.09 ± 15.06］%, P < 0.05), percentage of grade a+b sperm (［30.10 ± 18.43］% vs ［39.80 ± 17.50］%, P < 0.05) and sperm motility (［53.62 ± 21.56］% vs ［62.83 ± 20.47］%, P < 0.05). Age was correlated negatively with sperm motility (r = －0.20,P < 0.05), BF% negatively with the semen volume (r = －0.21, P < 0.05), the percentage of grade a sperm (r = －0.21, P < 0.05) and the percentage of grade a+b sperm (r = －0.18, P <0.05), and BMI negatively with the semen volume (r = －0.26, P < 0.01), percentage of grade a sperm (r = －0.23, P<0.01) and percentage of grade a+b sperm (r = －0.18, P < 0.05). Further multivariate analysis also showed that BF% was negatively correlated with the semen volume and percentage of grade a+b sperm after exclusion of age and other factors. CONCLUSIONS Obesity affects the semen volume, percentage of grade a sperm, percentage of grade a+b sperm and sperm motility in male infertility patients. And BF% can be used as an indicator in the diagnosis of obesity.