To determine antagonistic effect of Clostridium butyricum against Vibrio harveyi and its probiotic effect on growth performance, digestibility and immune response of fresh water prawn, Macrobrachium rosenbergii juveniles were examined following feeding with C. butyricum incorporated feed for 60 days. Significant reduction of V. harveyi growth was found at 8 hr and onward in in-vitro and at 10 days and onward in in-vivo challenge test. After rearing prawn with the bacteria in feed treatment for 60 days, body weight and growth rate of prawns was significantly higher (p < 0.05) than in control. Digestive protease and amylase activities in the gastrointestinal tract were also significantly (p < 0.05) higher than in controls. Immune response was a little high in treatment group, although it was not significant (p > 0.05) compared to control group. This study revealed that probiotic, C. butyricum incorporated diets were found to be beneficial for M. rosenbergii culture in terms of hindering the growth of pathogenic bacteria and increasing the growth, protease and amylase activities of prawn. Results from this study will be helpful to improve fresh water prawn farming.