Abstract: The present experiment was conducted to study the effect of soybean isoflavones (SI) on testis development, levels of plasma reproductive hormones, testosterone synthesis related enzyme and protein mRNA expression of young breeder rooster. One hundred and eight 10- week- old breeder roosters were randomly divided into 6 groups, each group had three repeats, and 6 roosters in each repeat. Corn-fishmeat diets was used as basal meal, and different doses of SI was added: 0 mg/kg, 5 mg/kg, 10 mg/kg, 15 mg/kg, 20 mg/kg, 25 mg/kg, respectively. The experiment lasted for 9 weeks, including one week pre- trial feeding. Plasma hormone (GnRH, T, LH, and FSH) were detected by collecting plasma at the end of the experiment. The testis samples were collected and weighed, the expression level of mRNA of testosterone synthesis related to both enzyme (P450scc, 3β-HSD) and protein (StAR,) was investigated. The results showed that: (1) among the dose range of 5-15 mg/kg, SI promoted the testicular development of young breed roosters, especially with the additive amount of 5 mg/kg, the testis weight and testis index were significantly increased (P＜0.01); with the dose of 25 mg/kg, testis index was significantly decreased (P＜0.01); (2) when the additive amount of SI was 5 mg/kg, the effect of promoting reproductive hormone secretion was most obvious; when the additive amount of SI was 25 mg/kg, the level of reproductive hormone secretion was decreased; (3) among the additive range of 5- 15 mg/kg, the level of StAR mRNA was significantly increased (P＜0.05), but there was no significant difference in the expression of P450scc and 3β-HSD (P＞0.05). In the present experiment, when the additive amount was 5 mg/kg, SI promoted the testicular development of young breed roosters and enhanced the expression amount of StAR mRNA and the level of reproductive hormone secretion. The safe additive range of SI was 5-20 mg/kg.