The high within-litter birth weight variation has become a big issue in multiparous animals. The present study was conducted to investigate the effects of leucine supplementation in premating diet on the reproductive performance, maternal antioxidative capability, and immune function in primiparous rats. Six-week-old female SD rats were assigned to basal diet or 0.6% leucine supplemented diet for two weeks. After mating during the eighth week of age, the rats were fed with regular gestation diet. Maternal blood samples were collected on the day before mating (day -1) and day 7 and day 20 of pregnancy, while ovaries and uteruses were obtained on day -1 and on day 7, respectively. The results indicate that, compared with control group, within-litter birth weight variation was significantly decreased, while birth weights were significantly increased in the leucine group (P < 0.01). Also, leucine improved the embryo distribution uniformity and the number of implantation sites in uterine. The ovarian gene expressions of LHR, CYP19A1, and VEGFA were upregulated, while Mucin-1 was decreased significantly (P < 0.05). Leucine also increased the maternal antioxidant capacity and immune function. Conclusively, leucine supplementation in premating diet could improve the reproductive performance, which could be attributed to the improved oxidative and immune status.