A protein-based hydrogel with tunable gelation time and mechnical strength was obtained by the hydrogelation of soy protein isolate (SPI) in the presence of microbial transglutaminase (MTGase). In order to control the gelation process and understand the relationship between the property and network structure of the formed SPI hydrogel, the changes of viscoelastic properties with time during the gelation process were monitored by the use of dynamic rheometry. The measurements were carried out at different protein concentrations, enzyme amounts, and reaction temperatures to clarify their effects on the gelation kinetics. In particular, the fractal characteristics of the SPI hydrogels formed in the presence and absence of MTGase were examined by relating the theological data to a scaling model. In addition, the resultant SPI hydrogel matrix was investigated for the controlled release of 5-aminosalicylic acid as the model drug.