Soy protein isolate (SPI) was grafted with gum acacia (GA) in this work. In comparison with classical heating at high water activity, ultrasound could accelerate the graft reaction between SPI and GA. A degree of graft (DG) of 34 was obtained by ultrasonic treatment for 60 min, whereas 48 h was required with classical heating. Ultrasonic treatment improved the concentration of available free amino groups of SPI. The grafted SPI showed significantly (p < 0.05) higher levels of emulsifying activity index, emulsifying stability index, and surface hydrophobicity than native SPI. The droplet size (D[3,2] and D[4,3]) of SPI emulsion decreased from 8.3 to 2.3 microm and from 25.7 to 7.1 microm through ultrasound-assisted grafting, respectively. Moreover, SPI-GA conjugates gave emulsions exhibiting more stability against creaming than those prepared with only SPI during ambient temperature storage (20 days). Decreases of lysine and arginine contents during the graft reaction indicated that these two amino acid residues attended the covalent linkage between SPI and GA. The results of secondary structure suggested that grafted SPI had decreased alpha-helix and beta-sheet levels and increased unordered coils level.