Many studies have been done on the effect of probiotics in aquaculture, but in the context of their effects on fish reproductive activities, few studies have been done. In this study, the effect of probiotic Primalac on some reproductive parameters in sword tail fish was examined. Female and male fish were fed 5% of body weight with the concentration of 0.0 (control), 0.4, 0.9 and 1.4 g primalac kg^-1 of dry diet (twice a day) for a period of 3 months. At the end of the experiment gonadosomatic index, the number of larvae per female, relative fecundity, survival rate and the percentage of defective larvae were evaluated. Results showed that relative fecundity and gonadosomatic index in treatment 0.4 g was significantly higher than other treatments (P<0.05). The highest and lowest survival rate was observed in treatment 0.4 g and control respectively. Moreover, the highest defective larvae were found in control group. Totally, results showed that probiotic Primalac increased the reproductive performance of sword tail fish.