To elucidate the effects of soybean protein and casein on postprandial lipemia, oral fat load tests were performed before and 3 weeks after the administration of soy protein isolate (SPI) and casein supplement to normolipidemic men. Eleven normolipidemic male subjects on otherwise identical controlled diets were assigned to either a 20 g/d soy protein isolate (SPI) dietary supplement or a casein dietary supplement for three weeks in a crossover design. Fat load tests with 40 g/m2 of bovine milk fat were carried out before and after 3 weeks on the experimental dietary supplements. Fasting plasma concentrations of lipids and apolipoproteins were not significantly different from baseline levels before or after the administration of SPI or casein supplemented diets. Neither SPI nor casein supplement affected the fasting plasma concentrations of lipids and apolipoproteins. The areas under the incremental curve (AUIC) of triglyceride (TG) and remnant-like particles triglyceride (RLP-TG) after both experimental diets were not significantly different from those before the experimental diets. However, the AUIC of remnant-like particles cholesterol (RLP-C) showed a tendency (p = 0.07) to decrease after administration of the diet supplemented with SPI than before the diet. The AUIC of RLP-C was significantly (p < 0.05) lower after the diet supplemented with SPI than after administration of the diet supplemented with casein. These results suggest that 3 weeks of 20 g/d SPI dietary supplement favorably affects the postprandial remnant lipoprotein response as compared to the casein dietary supplement.