This study was performed to investigate effects of dietary fat content and degree of saturation on the function of the immune system. Sixty male BALB/c mice average-weighing 17g were divided into three dietary groups: 5% safflower oil group, 20% safflower oil group, 19.3% beef tallow & 0.7% safflower oil group. Food intake and body weight were measured every day. At 4th, 7th, 10th week after dietary treatment, organ weight measurements, delayed-type hypersensitivity test, plaque forming cell test, agglutination test, differential white cell count and histological examination of spleen were performed. Results are follows; 1) Body weight, food intake and calorie intake were not different in the three dietary groups during the experimental period($alpha$=0.05). 2) Liver weight was significantly higher in 5% safflower oil group($alpha$=0.05). Spleen index was slightly higher in mice fed 5% safflower oil and 19.3% beef tallow & 0.7% safflower oil. Thymus index in all mice was decreased by aging. 3) Delayed-type hypersensitivity of the mice fed 5% safflower oil and 19.3% beef tallow & 0.7% safflower oil was significantly higher than that of the mice fed 20% safflower oil. 4) The number of plaque forming cell was significantly reduced at 10th week compared to 7th week in all group($alpha$=0.05). Although there was no difference in plaque forming cell among three groups at 10th week, 5% safflower oil group showed slightly higher plaque forming cell than 20% safflower oil group at 7th week. 5) At 4th week, agglutination test seems to be higher in 5% safflower oil group and 19.3% beef tallow & 0.7% safflower oil group compared to 20% safflower oil group. 6) Percentage of neutrophil and eosinophil was slightly reduced in 20% safflower oil group. 7) Spleen tissue was not affected by and dietary treatments. According to our results, the higher the fat content & unsaturation of the diet the lower the cell-mediated immunity of the mice. Humoral-immunity did not appear to be affected by the dietary manipulation. However humoral-immunity was decreased significantly by aging in all dietary groups.