In this paper, composite nanoparticles consisting of soybean peptide aggregates (SPA) and (-)-epigallocatechin gallate (EGCG) were successfully prepared by ultrasonic treatment and its particle size, zeta potential, fluorescence spectrum and X-ray diffraction pattern were measured. Furthermore, the droplet size and oxidation stability of emulsions stabilized with SPA or the composite nanoparticles were evaluated. The results showed that ultrasound and EGCG had a synergistic effect in reducing particle size. The nanoparticles with 0.80% (m/m) EGCG had the smallest particle size and the largest absolute value of zeta potential. After addition of EGCG, nanoparticles changed from a crystalline to an amorphous state, and its fluorescence intensity was quenched in a EGCG concentration-dependent manner. Compared with the SPA stabilized emulsion, the emulsifying activity, emulsion stability and oxidation stability of the emulsion stabilized with the composition nanoparticles were improved.