Abstract: Glutaminase (EC 3.2.1.5) was applied in the present work to treat soybean protein isolates (SPI) for deamidation and hydrolysis to a limited reaction extent. SDS-PAGE and size exclusion chromatography analysis were used to evaluate the degradation of SPI. Some deamidated SPI products with lower degree of deamidation were prepared with reaction conditions of SPI concentration of 5% (w/v) , glutaminase addition level of 400U/kg SPI, reaction temperature of 37℃ and reaction time were 12, 24 and 36h, respectively. Three deamidated SPI products prepared had the degree of deamidation of 3.5% , 5.6% and 6.6% , or degree of hydrolysis of 0.9%, 2.7% and 3.5%, respectively, were prepared. The iron (II) chelating activities and scavenging activity on DPPH radical of three deamidated SPI products were analyzed. The results indicated that the iron (II) chelating activities and scavenging activity on DPPH radical of three deamidated SPI products were higher than that of SPI, and would be enhanced as degree of deamidation increased.