Consumption of thermoxidised and photoxidised palm oil diets is known to be harmful to tissues in the body including the reproductive system. It is not known which of these two forms of the commonly consumed vegetable oils is less harmful to reproductive function. Fifteen male Wistar rats weighing 80-125g and aged 19 to 23 weeks were randomly divided into control, thermoxidised palm oil (TPO) and photoxidised palm oil (PPO) diets-fed groups of five rats per group. Control group was fed on plain rat feeds while the other two groups were fed on TPO diet or PPO diet accordingly for 13 weeks. Results demonstrated a significantly lower seminal pH in the PPO-fed than in the TPO-fed group (P<0.01) and control (P<0.05). Sperm motility was significantly reduced in the PPO-fed compared with TPO-fed groups (P<0.05). The viability of sperms was significantly decreased in PPO-fed rats compared with the TPO-fed group (P<0.05) and control (P<0.01). Sperm count was significantly lower in PPO-fed compared with TPO-fed group (P<0.01). Percentage of sperms with morphological defects was significantly higher in the PPO- than in the TPO-fed groups (P<0.01). Serum testosterone concentration was significantly decreased in PPO-fed compared with TPO-fed group (P<0.001). Serum luteinizing hormone (LH) level was significantly decreased (p<0.001) in PPO-fed group compared with control and TPO-fed groups. Serum concentration of follicle stimulating hormone (FSH) was significantly decreased (p<0.001) in PPO-fed group compared with control and TPO-fed groups. In conclusion, chronic consumption of PPO diet has greater deleterious effects on male reproductive function than TPO diet