The aim of the current study was to investigate the effect of supplementation of diet with extruded soybean on performance and reproductive traits of Holstein bulls. Sixteen Holstein bulls (body weight 200 ± 10 kg; 8 month of age, two treatments each with eight replicates) were randomly assigned in a completely randomized design test. Bulls were fed with iso-energetic and iso-nitrogenous for 10 months. In order to investigate dry matter intake, the last 20 days of experiment were allocated to estimate dry matter intake. In order to investigate the ruminal fermentation, rumen fluid was collected at the end of the experiment. Scrotal circumference of each bull was measured at the beginning and end of the experiment. In order to investigate sperm quality, three times semen obtained from each bull using an electro-ejaculator. Semen volumes were recorded then evaluated by CASA. Dry matter intake were not affected by treatments. Total VFA, acetate, propionate, butyrate, acetate to propionate ratio, pH and ammonia nitrogen were the same between treatments (P 0.05). Scrotal circumference was increased significantly (P 0.05) in the diet containing extruded soybean. Dietary supplementation with extruded soybean significantly increased total motility, progressive motility, fast movement, average path velocity and curvilinear velocity (P 0.05). Moreover, immotile sperm significantly decreased in extruded soybean treatment (P 0.05). Semen volume and sperm output increased significantly in extruded soybean but, sperm concentration were not affected. In conclusion, it seems dietary supplementation with extruded soybean as a source of polyunsaturated fatty acid in Holstein bulls can improve reproductive performance in terms of scrotal circumference growth and qualitative sperm parameters. Keywords : Reproductive traits, Scrotal circumference, Total motility.