Abstract: This study was conducted to evaluate the immunostimulatory and growth promoting effects of four different herbal medicinal plants on non-specific immune response and resistance to Aeromonas hydrophila challenge of common carp, C. carpio. The ethanol extract of 11 plants were screened for their antimicrobial activity against A. hydrophila, a bacterial pathogen. Of these, 4 plant extracts, Ocimum basilicum, Cinnamomum zeylanicum, Juglans regia and Mentha piperita were selected and mixed thoroughly in equal proportion. The mixed extract was incorporated with the artificial feeds at concentration of 0.0 (A), 250 (B), 500 (C), 750 (D), 1000 (E) and 1250 (F) mg kg^-1 of dry diet and fed to healthy fish. After 45 days of feeding, a challenge trial was conducted by injection of A. hydrophila for 10 days. Every 15 days and also at 10th day post-challenge, immunological, biochemical and haematological parameters of fish were studied. Results indicated that fish fed with herbal immunostimulants diets showed enhanced bactericidal activity, serum lysozyme, respiratory burst activity, WBC, RBC, haemoglobin, total serum protein, albumin and globulin compared to the control group (p<0.05). As the value of herbal extracts was increased in diets, the plasma glucose level decreased. The mortality was recorded up to 10 days post-challenge. All experimental groups showed higher survival rate compared to control (p <0.05). The survival percentage was found highest (91.42%) in the group E and lowest (48.58%) in control group. It can be conducted that the plant extracts we used in this study can act as immunostimulants, enhance the non-specific immunity and increase disease resistance of common carp, C. carpio to A. hydrophila infection.