Introduction: Sperm cell membranes are susceptible to peroxidative damage by an excess of reactive oxygen species (ROS). Antioxidative defence systems consisting of glutathione peroxidase (GSH-Px), superoxide dismutase (SOD) and catalase (CAT) physiologically control the balance between ROS production and neutralization. Materials and Methods: In the present study, correlations between seminal plasma glutathione peroxidase enzyme activity and Abstract of the 8th Royan International Twin Congress, Tehran, Iran, 5-7 September 2007 12 Yakhteh Medical Journal, Vol 9, Sup 1, Summer 2007 semen parameters are evaluated in 240 males. Semen analysis was performed according to World Health Organization guidelines. The 240 males were subdivided into 5 groups as normospermia, oligozospermia, asthenozospermia, azospermia and varicocele according to their spermiogrames. Seminal plasma glutathione peroxidase enzyme activity was determined by Kit (Randox, Germany). Results: The result showed that glutathione peroxidase enzyme activity is higher in normospermic than oligozospermia, asthenozospermia, azospermia and varicocele groups. Also, there are significant and negative correlations between glutathione peroxidase enzyme activity and seminal plasma fructose concentration, white blood cell, tail defects of sperm, coiled tail sperms and short tail sperms. On the other hand, the present data showed that significant and positive correlations between vitality, sperm count, motility and normal morphology. Conclusion: So, the present study showed that measurement of glutathione peroxidase enzyme activity could be a good marker for evaluation of male infertility.