The purpose is to determine the optimal size of gelatin sponge, less toxic and more effective, in transcatheter arterial embolization (TAE) of the liver. In experimental study on toxicity, gelatin sponge particles (GSP) were divided by their size into four groups, 0-200 microns, 200-500 microns, 500-1000 microns and 1000-2000 microns. In each group, three mongrel dogs underwent TAE. After a week, the livers were investigated pathologically, and the rate of liver necrosis was measured. The average rate was 23.7 percent with GSP of 0-500 microns, while 0.5 percent with 500-2000 microns. Microscopically, coagulation necrosis was found in 6 of 6 dogs with 0-500 microns, while in 2 of 6 with 500-2000 microns and bile duct injury was found in 5 of 6 dogs with 0-500 microns, while none with 500-2000 microns. In clinical study on effectiveness, ten patients with hepatocellular carcinomas underwent TAE with GSP of 0-500 microns, 500-1000 microns and 1000-2000 microns respectively. After a week, the rate of tumor necrosis was measured by CT. The average rate was 94.8 percent with 0-500 microns, 93.5 percent with 500-1000 microns and 86.8 percent with 1000-2000 microns. These results suggest that the optimal size of GSP is 500-1000 microns.