In this study, the effects of incorporating soy protein isolate (SPI) and its hydrolysates (SPHs) on the gel properties of high-protein soy yogurt was investigated. Papain hydrolysis induced increased SPI solubility, reduced molecular weight and a more uniform size distribution as indicated by SDS-PAGE and SEC-HPLC. Formation of SPH aggregates were proven by increased particle size (4 μm) and decreased total free SH content and surface hydrophobicity. Soy yogurt with SPH addition at an appropriate degree of hydrolysis (DH; 1 to 3 %) exhibited reduced large protein aggregates and surface roughness, as observed by CLSM and SEM. Rheological and tribological analyses indicated that addition of SPHs reduced viscoelasticity and friction coefficient. However, excessive DH (4 to 6 %) resulted in low viscoelasticity that was difficult to maintain in a gel-like state, probably due to interference from preformed SPH aggregates and enhanced inter-protein electrostatic repulsion induced by raised pH and lowered isoelectric point.