Background: The effect of L-carnitine (LC) on the male infertility have been previously reviewed. However there are few studies about the effect of LC on sperm function parameters in vitro. Objectives: The objective of this study was to improve certain sperm function parameters in vitro of asthenozoospermic infertile men using LC as a motility stimulant substance which may enhance the medium used for assisted reproductive technologies (such as artificial insemination) in future. Methods: Semen was collected from 50 infertile men involved in the current study. Each semen sample was divided into two portions. One part was considered as a control and in vitro activated by using culture medium only. The other portion was considered as treated group and in vitro activated by adding LC (0.5mg) to the culture media. Certain sperm function parameters were examined before and following in vitro activation using simple layer technique. Results: The results revealed highly significant increment (P<0.01) in the percentage of total sperm motility grades (A+B+C) and progressive sperm motility grade (A) with a significant improvement (P<0.05) in the percentage of progressive sperm motility grade (B) and morphologically normal sperm when using LC medium in comparison with control medium after 30 minutes incubation. Conclusion: It is concluded from the results of the present study that adding 20% LC to the culture medium ofthe in vitro sperm activation leads to an improvement in certain spermfunction parameters.