Abstract: 【Objective】 The purpose of this experiment is to study the effects of supplementary L-citrulline on the semen quality and the anti-oxidation ability of seminal plasma and plasma of stallion. 【Methods】 Twelve stallions at the age of 5-8 years were selected and randomly divided into control and trial group with 6 horses in each group. All horses were bred in the same environment and fed the same nutrient level diet, the trial group horses were fed 20 g/d L-citrulline. 【Results】 (1)Compared with the control group, supplementary L-citrulline significantly increased the amount of sperm collected and the linear rate of sperm movement by 28.08% (P＜0.01) and 19.12% (P＜0.01), respectively. (2) L-citrulline could significantly improve the SOD activity and inhibit the hydroxyl radical activity of the seminal plasma (P＜0.01), significantly increased the viability of GSH-PX (P＜0.05), and significantly reduce the concentration of MDA (P＜0.01). (3) Supplementary L-citrulline could significantly increased the activity of CAT, GSH-PX, T-AOC and inhibit hydroxyl free radicals of plasma (P＜0.01), and significantly reduce the concentration of MDA (P＜0.01). 【Conclusion】 Therefore, supplementary L-citrulline fed to the breeding stallions can improve their semen quality and the anti-oxidation ability of the sperm and plasma of stallion.