Many clinical studies have reported that fish oil supplementation has beneficial effects supporting the idea that n-3 polyunsaturated fatty acids in fish oil are anti-inflammatory and immunomodulatory. Sixty one-day old broiler chicks were used in this study to determine the effect of fish oil on humoral immunity. They were divided into 3 equal groups (A, B &C) and raised under the same conditions until the end of the study at 49 day of age. The birds were vaccinated 3 times against Newcastle disease, at the 7th day they were vaccinated with Hitchner B1 and at 21st day and 35th day with LaSota type vaccine. Group A was fed fish oil supplemented diet for 14 days before the 3rd vaccination, whereas group B was fed after vaccination. Group C was served as control. Serum samples have been collected at 49th day from all groups. Antibody titer was detected by HI test; 2-mercaptoethanol-sensitive IgM and 2-mercaptoethanol-resistant IgG were also measured. Serum total protein, albumin and globulins were also estimated. Differential white blood cell count was performed to detect the number of heterophils and lymphocytes and their ratio. The results indicated that HI antibody titer was significantly ( p < 0.05 ) increased in both group A and B. Two-mercaptoethanol-sensitive IgM and 2-mercaptoethanol-resistant IgG were also showed significant ( p< 0.05 ) increase of IgG in both groups. The present study was also revealed a significant (p< 0.05) increase of serum globulins of group A and B in comparison to that of control group. Heterophils and lymphocytes percentage of both treated groups (A and B) significantly higher than that of untreated control group. These results showed that 50 gm/kg of diet fish oil accelerates antibody production and maintain proper immune function in chickens fed after vaccination against Newcastle disease with LaSota type vaccine at 35th day of age.