Effects of ${alpha}$-chymotrypsin modification on degree of hydrolysis (DH), solubility, emulsifying capacity and thermal aggregation of laboratory-purified soy protein isolate (SPI) using a lipoxygenase-defected soybean (Jinpum-kong) and commercial soy protein isolate (Supro 500E) were compared. SPIs were hydrolyzed by ${alpha}$-chymotrypsin at pH 7.8 and $37^{circ}C$ for 30 min. DHs of Supro 500E and Jinpum-kong SPI were increased by ${alpha}$-chymotrypsin modification, and DH of Supro 500E was higher than that of Jinpum-kong SPI. DH of ${alpha}$-chymotrypsin treated Jinpum-kong SPI was similar with untreated Supro 500E and DH of treated Supro 500E was the highest. Solubility, emulsifying capacity and thermal aggregation of SPIs were increased by ${alpha}$-chymotrypsin modification, and these changes were highly related to changes in DH. Functional properties of Supro 500E were higher than Jinpum-kong SPI in both of untreated and ${alpha}$-chymotrypsin treated SPIs.