The effect of dietary supplementation of fish oil on performance and gut health of broiler chickens was examined. Two hundred one-day old broiler chicks were randomly allotted to 4 treatments with 5 replicates of 10 birds each in a completely randomized design. Diet 1: basal diet without feed additives. Diet 2: basal + coccidiostat (0.06%), Diet 3: basal + 2.5% fish oil and Diet 4: basal + 5% fish oil. Performance indices were measured. On day 56, distal ileum of two birds per replicate were severed for gut morphological measurements and digesta samples were collected for protozoan load counts. About 5cm of the jejunum, ileum and caeca were excised for histological examinations.   The results showed that dietary fish oil supplementation did not have significant effect on the performance of broiler chickens at the starter phase. However, there was a negative effect of the fish oil in the final weight and feed conversion ratio of birds at the finisher phase. Final weights of birds that received the basal diet (1736g/b) were significantly higher (P< 0.05) than those placed on 5% fish oil supplemented diet (1556.3g/b). Crypt depth and villus height of birds fed with 5% fish oil supplemented diet were significantly higher (P<0.05) than those on the other diets. Birds on the control diet had numerous coccidia while fewer coccidia were observed in birds fed coccidiostat, 2.5% and 5% fish oil diets. Photomicrographs of the jejunum, ileum and caeca of birds fed the basal diet showed numerous developing stages of coccidia in the degenerated enterocytes, loss of villi and cryptal degeneration. Those on coccidiostat diet showed fusion, atrophy of villi and necrosis of mucous cells. However, in birds fed 2.5% and 5% fish oil supplemented diets; mild expansion of lamina propria and normal mucosa with no observable lesions were recorded. In conclusion, dietary supplementation of 2.5 or 5.0% fish oil resulted in adverse effect on broilers performance (body weight and FCR) accompanied with slight improvement of gut health.