The efficacy of fish oil (FO) on serum biochemical parameters and performance was investigated in streptozotocin (STZ)-diabetic rats. Four weeks before the start of FO treatment, diabetes was induced in rats by a single intraperitoneal injection of streptozotocin (STZ = 60 mg/kg bodyweight). Then, male Wistar rats (n = 32) were randomly divided into 4 groups of non-diabetic (T1, control), treated non-diabetic (T2), diabetic (T3), and treated diabetic (T4). Standard rat non-purified diet was fed either alone or supplemented with an additional 2.5% fish oil. Diabetic rats had significantly increased serum levels of cholesterol, triglycerides, glucose and low density lipoprotein cholesterol (LDL-C) and decreased serum high DL-C concentration. The fish oil treatment of diabetic rats resulted in significant recovery in bodyweight, heart weight: bodyweight ratio and blood glucose levels. The administration of FO reduced cholesterol, triglyceride and LDL-C levels, and increased HDL-C levels, in the serum of diabetic rats. In conclusion, fish oil can reduce the risk of cardiovascular disease in diabetes with a significant improvement and regulation in lipid metabolism and also can moderate glucose level of diabetic rats and regulate its secretion that may be reduce of type 1 diabetes mellitus.