This study investigated the Onion Powder (OP) supplementation in the diet of the goldfish (Carassius auratus) on the blood parameters during eight weeks. Fish averaging 1.16 ± 0.09 g were randomly stocked into aquarium. Four experimental diets were prepared to contain onion powder at the concentrations of 0, 0.5, 1 and 1.5% respectively. In order to adapt the fish to experimental condition at the end of trial blood indices, WBC, RBC, Hb, and HCT were determined. Haematocrit was significantly (p<0.05) higher in the group fed onion powder 0.5% and 1% compared to the control group. In conclusion, it can be suggested that dietary inclusion of 0.5% onion powder was effective at improving hematological parameters of goldfish.