Objective: To measure the concentrations of folate, vitamin B 12 (VB 12 ),and the levels of reactive
oxygen speciees (ROS) in whole semen, and to determine their relationships with various sperm parameters. Methods: The semen from 176 healthy fertile men were collected and analyzed according to WHO guidelines. The concentrations of folate and VB 12 as well as levels of ROS were determined.Results: The means of sperm volume, density, and motility for the study population were generally within the normal range of WHO criteria for sperm parameters. The mean concentrations of folate and VB 12 were 20.09 nmol/L and 667.73 pmol/L respectively. The mean level of ROS was 71631 RLU/5 minutes. The concentration of VB 12 showed a significantly positive relationship with sperm density (r=0.31,P<0.001). A statistically negative relationship was noted between VB 12 and the percentage of morphological defective sperm(r=-0.15,P=0.048).VB 12 also showed an inverse correlation with the level of ROS(r=-0.25,P=0.002).There was a significantly negative correlation between the concentration of folate and the levels of ROS (R=-0.25,P=0.003).The level of ROS showed a significantly positive correlation with sperm motility (r=0.19,P=0.019) and with the percentage of morphological defective sperm (r=0.25,P= 0.002 ). Conclusion: VB 12 is essential for spermatogenesis and maintenance of normal sperm functions. The level of ROS is closely associated with the morphological defect of sperm. Both folate and VB 12 are important for normal sperm functions, perhaps by reducing semen ROS.