The influence of increasing number of mating partners on the copulatory behaviour and reproduction in Brandt’s voles (Microtus brandti) was studied. Compared with the control group of 1 ♂ + 1 ♀, our results showed when increasing only female partners, the mating opportunity was increased in males while decreased in female in the treatment group of 1 ♂ + 2 ♀♀; the mating opportunity of male and female were both decreased significantly in the treatment group of 1 ♂ + 3 ♀♀. When increasing only male partners, male mating opportunity in treatment groups 2 ♂♂ + 1 ♀ and 3 ♂♂ + 1 ♀ decreased significantly while no change was significant in females in both treatment groups. When increasing the number of male and female partners simultaneously in the treatment group of 2 ♂♂ + 2 ♀♀, the mating opportunity for both males and females was decreased significantly. Comparing with the control, the mean litter size in all five-treatment groups lessened significantly and female pregnancy rate dropped significantly if they copulated less than five times. From these results, we have demonstrated a significant reproductive interference due to mate competition in Brandt’s voles and thus, competitively reproductive interference may be an important behavioural mechanism in regulating population density in this rodent species. However, due to possible laboratory artifacts, we still need further study, especially in the field, to validate our results.