In order to reduce the bitterness of soy protein hydrolysates, chemical modification of lysine wich had amino group was investigated. Isolated soy protein (ISP) was treated with N-acetylimidazole that specifically acetylated lysine and tyrosine. To have effects of chemical modification only on lysine, O-acetyl tyrosine was deacetylated at pH 11. The lysine-acetylated ISP and control ISP were hydrolyzed by bromelain to the same degree of hydrolysis (10\%), then the bitterness of each protein hydrolysates was evaluated. Sensory analyses indicated that the bitterness of hydrolysates of lysine-acetylated ISP decreased in comparison with that of hydrolysates of control ISP. Surface hydrophobicity of hydrolysates of lysine-acetylated ISP increased, but they had fewer lysine residues at the C-terminal region than those of control ISP. Consequently, the acetylation of lysine decreased the bitterness of protein hydrolysates, which implied that the amino group of lysine played a role for the sensation of bitterness.