The present study aimed to investigate effects of dietary 1,8‐cineole (cineole) on common carp (Cyprinus carpio) health under ammonia toxicity. The fish were fed with four diets supplemented by 0 (control), 0.1 (0.1C), 0.5 (0.5C), and 1 (1C) percent cineole for 2 weeks, before a 24‐hr ammonia exposure. Blood samples were taken before and after ammonia exposure. The results showed that ammonia exposure significantly (p < 0.05) increased serum urea levels, and decreased serum alternative complement (ACH50) activity and serum total immunoglobulin (Ig) levels. Cineole significantly (p < 0.05) increased serum ACH50 activity (0.1% and 0.5% levels) and total Ig levels (0.5% levels). There was no significant difference (p > 0.05) in serum total protein, albumin and globulin levels among the treatments. Before ammonia exposure, there was no significant difference (p > 0.05) in serum alkaline phosphatase (ALP), alanine transaminase (ALT), and lysozyme activities among the cineole treatments. The treatment 0.5C had the lowest lactate dehydrogenase (LDH) and aspartate transaminase (AST) activities before ammonia exposure. After ammonia exposure, serum LDH and ALT activities increased in all treatments, but the treatment 0.5C had the lowest activities. Serum ALP activities increased after the ammonia exposure in the control and 1C groups. Serum AST and lysozyme activities increased after the challenge in all groups except for the 0.5C group. Overall, the results suggest that ammonia exposure causes increase in serum LDH, ALP, AST, and ALT activities and decreased immune responses, which are indicator of tissue damage and immunosuppression. 0.5% cineole is capable to suppress adverse effects of ammonia intoxication in carp.