The probiotic, Lactobacillus rhamnosus (LAC-R) and Betaine (BT) alone and combined were used in the diet of the common carp, Cyprinus carpio to evaluate their potentials in improving growth, immunity and disease resistance. Seven experimental treatments (LAC-R: 1 × 108 CFU/g LAC-R; BT1: 0.5 % BT; BT2: 1 % BT; BT3: 2 % BT; LAC-R + BT1; LAC-R + BT2; LAC-P + BT3) and one control group (control: non-supplemented fish) were designed in three replicates and fish (13.3 ± 1.5 g) fed with the test diets for two months. After the feeding experiment, fish were exposed to Streptococcus iniae for 14 days to evaluate the fish disease resistance. The fish supplemented with BT and LAC-R alone or combined exhibited better growth compared to control (P<0.01), however, fish of LAC-R + BT had the highest growth performance (P<0.01). Digestive enzyme activities increased in response to LAC-R, BT and their combinations, in which the highest activities were for LAC-R + BT2 and LAC-R + BT3 (P<0.01). The lactic acid bacteria (LAB) populations in intestine increased after the probiotic supplementation (P<0.01), while the results were non-significant for total bacteria count (P>0.01). The total antioxidant capacity (T-AOC), antioxidant enzymes [serum catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx)] and immune components [lysozyme, total immunoglobulin (Ig), myeloperoxidase (MPO), alternative complement activity (ACH50), Nitroblue tetrazolium (NBT) and total protein in blood and total Ig, lysozyme, alkaline phosphatase (ALP) and protease in mucus] were mostly enhanced by LAC-R, BT2, BT3 and their combinations (P<0.01). The oxidative stress was ameliorated by LAC-R, BT and their combinations, as the levels of malondialdehyde (MDA) were lower in these treatments (P<0.01). Fish supplemented with LAC-R, BT and LAC-R + BT had lower mortality rate compared to control after challenge with S. Iniae (P<0.01). In conclusion, dietary LAC-R and BT improved growth and immune performance in the fish. Also, the best performance was found for fish supplemented with a combination of LAC-R and 1–2 % BT.