This study aims to evaluate the possible improving effects of fish oil, olive oil and melatonin on the induced hypercholesterolemia in adult male rats. 50 rats were used in this study and were divided into 5 groups 10 rats each. Rats of group 1 were fed on a standard diet and those of group 2 were fed on a standard diet enriched with 1% cholesterol (cholesterol fed group) for 10 weaks. Groups 3-5 were fed as in group 2 then the diet was replaced by standard diet and fish oil in group 3 (fish oil group), standard diet and olive oil in group 4 (olive oil group) and standard diet and melatonin in group 5 (melatonin group) for 2 weeks . Then, blood samples were taken from all animals and the aorta of each animal was obtained after slaughtering and examined histologically to assess the presence of atherosclerosis. Parameters of the lipogram [total plasma cholesterol (TPC), high density lipoprotein (HDL), low density lipoprotein (LDL) and triglycerides (TG)], superoxide dismutase (SOD), total thiol, nitric oxide (NO) and lipid peroxide (LP) were measured. Feeding cholesterol significantly increased TPC, LDL, TG and LP and significantly decreased HDL, SOD, NO and total thiol. There was a significant decrease in TPC, LDL, TG and LP by using fish oil, olive oil and melatonin while, the level of SOD, NO and total thiol were significantly increased and non significant increase in the level of HDL. Fish oil caused the greatest reduction in TG and the greatest increase in NO denoting improvement of vascular endothelial function. Olive oil was the most effective in reducing TPC and total thiol and melatonin was the best factor reducing LDL and LP and consequently atherogenesis and was the most effective in restoring SOD. Histological examination of the aorta from rats of the fish oil , olive oil and melatonin groups showed atheromatous fibrous plaques nearly to the same extent in all groups but absence of well developed fibrous cap which was found in the cholesterol fed group denoting slight improvement. It was concluded that diet additives as fish oil, olive oil or melatonin injection have modulating effect on the parameters of the lipogram, oxidative stress markers and histological features of atherosclerotic lesion and that the improvement of the aortic wall was slight due to the short period of treatment (2 weeks only) to produce marked change in the aortic wall.