This study aimed to determine the effect of dietary supplementation of coconut oil levels on some physiological and immunological responses in ducklings during growing period (28-89 day of age). A total number of 120 Domyati ducklings, 28- day old were used, weighted 530-545 g and divided into four experimental groups, each of three replicates to determine the effects of feeding diets contained different levels of coconut oil (CO) as a medium chain fatty acids (MCFA) source. The experimental groups were arranged as follows: The first as a control group (G1) which received basal diet without any supplementation, while the second, third and fourth groups (G2, G3 and G4) were received diet contained CO by 1.0, 1.5 and 2.0 %, respectively. The results revealed that, percentages of heterophils and lymphocytes were significantly (P≤0.01) lower and higher, respectively, for Domyati ducklings in G4 than those in the control group (G1). Some blood hematological values (Hb and RBCs) did not significantly affected by different CO levels. The ratio of hetrophil to lymphocytes (H/L) was significantly (P≤0.01) lower for Domyati ducklings in G2 and G4 than those in G1. Spleen and thymus indexes were significantly (P≤0.05) increased in treated groups compared to the control one. Moreover, dietary CO levels caused to increase the values of plasma IgG, IgM and IgA concentrations than that of the control group. The different dietary levels of CO significantly influence the lipid peroxidation (LPO) by decreasing MDA values. Conclusively, ducklings fed diet supplemented with 1.5 or 2% CO had heavy immune organs weights, and high plasma levels of immunoglobulins, which collectively suggested as an improvement in the immune response.