Abstract: A feeding trial was conducted to investigate the effects of administration of microencapsulated probiotic Bacillus sp. NP5 and prebiotic mannan oligosaccharide (MOS) for the prevention of Aeromonas hydrophila infection on Pangasianodon hypophthalmus. Pangasius with initial body weight of 6.54±0.17 g were stocked into 40 L aquariums with a stocking density of 10 fish per aquarium. Control (C) diet (no addition of Bacillus sp. NP5 and MOS), probiotic Bacillus sp. NP5 1% (10 g kg–1), prebiotic MOS 0.2% (2 g kg–1) and synbiotic (probiotic Bacillus sp. NP5 1%+prebiotic MOS 0.2%). Then 10 fish of each aquarium were challenged by Aeromonas hydrophila. The results showed that fish with synbiotic supplementation resulted the highest Specific Growth Rate (SGR), Food Conversion Ratio (FCR) and those were significantly different (p<0.05) from the other treatment. The immune responses showed that fish fed with the control diet resulted the lowest hematocrit, hemoglobin and red blood cell count and those were significantly different from the probiotic and prebiotic group (p<0.05). White blood cell count, phagocytic activity and respiratory burst activity of the fish fed with probiotic 1% and MOS 0.2% were not significantly different from control. Moreover, fish fed with diet supplemented with probiotic, prebiotic and synbiotic had notably lower mortality after 10 days infected with A. hydrophila (p<0.05). Dietary Bacillus sp. NP5 and MOS had a significant interaction on enhancing immune responses and growth performances of Pangasianodon hypophthalmus (p<0.05).