Objective:To assess the effects of dietary L-Arginine (L-Arg) supplementation on testes histological parameters and semen quality of male broiler breeder.Methods: Twenty male broiler breeders at 37 wk of age were distributed in a completely randomized design with 4 treatments and 5 replicates each. The treatments consisted of 4 dietary levels of L-Arg (0.00, 1.35, 2.33 and 3.22 g/kg of the diet) and fed to birds for 8 wk. At the end of the experiment semen samples were collected by abdominal massage to determination of semen quality. The blood samples were used to determination of serum testosterone concentration at the end of the experiment. Furthermore, testes from each rooster were used to evaluation of their weights and histological parameters.Results:The results of the experiment showed that dietary L-Arg at 2.33 g/kg improved testes weight, semen volume and sperm forward motility in roosters (P<0.05). Besides, serum concentration of testosterone was increased in roosters fed 2.33 g/kg L-Arg (P<0.05). The results of testes histology indicated that seminiferous tubules lumen diameter, leydig cells, spermatides and sperm cells counts were greater in birds received 3.22 g/kg dietary L-Arg (P<0.05). However, the birds fed diet supplemented with 2.33 g/kg L-Arg had greater seminiferous tubules diameter, sertoli and spermatogonia cell counts than other groups (P<0.05).Conclusion:According to the results of this experiment, it is concluded that dietary L-Arg had positive effects on reproductive traits in roosters.