This study was conducted to investigate the possible protective effect of α-lipoic acid (ALA) supplementation to extender on damages in the quality of bull semen elicited by freze-thawing process-induced oxidative stres. Ejaculates were collected via an artificial vagina from the bulls at once a week. Then they were split into five aliquots and extended with the Tris base extender containing different doses of ALA, except control. The extended samples were equilibrated slowly to 4°C for 4 h and then frozen using a digital freezing machine. Frozen straws were thawed to analyse progressive motility and sperm motility characteristics as well as plasma membrane integrity. Biochemical assays were performed in a spectrophotometer using commercial kits. Treatment of extender with ALA groups caused a significant decrease in percentages of post-thaw sperm CASA, progressive motilities and sperm motility characteristics such as VAP, VSL and VCL except ALA0.5. Besides ALA0.5 showed lower percentages of acrosome and total abnormalities in comparison to the control. In conclusion, findings generated here showed that ALA0.5 supplementation in Tris based semen extender was of great beneficial effect on frozen-thawed bull semen in terms of morphology and plasma membrane integrity.