In order to investigate the effect of riboflavin on cholesterol and MLHP (methyl linoleate hydroperoxide) -induced fatty liver electron microscopically, riboflavin was given to rate receiving high cholestero and MLHP diet for 4 weeks, 8 week old male Sprague-Dawley rats were administrated diets daily, as following group ; usual diet (control ), riboflavin 910 ug/20gm BW), cholesterol (50mg/gm BW), MLHP(0.1ml/20gm BW), cholesterol plus MLHP, cholesterol plus riboflavin, cholesterol plus MLHP plus riboflavin, by means of intubation into the stomach for 4 weeks. Each group consisted of 10 rats, The liver of the animals were examined ultrstructurally by transmission electronmicroscope. The results wee as follows ; 1) The three group including cholesterol, MLHP, cholesterol plus MLHP feeding resulted in fatty liver. 2) The main finding of the fatty liver were swelling and vacuolation of mitochondria, a lot of lipid droplets, disarrangements and loss of rough endoplamic reticulums. 3) the most striking features in the group of cholesterol plus riboflavin, cholesterol plus MLHP plus riboflavin feeding were decrease of size and number of lipid droplets, mitochondrial swelling and vesiculation, with restortion of rough endoplasmic retriculums. It is postulated with above findings that riboflavin was effective in prevention of cholesterol and MLHP -induced fatty liver.