This thesis focuses on the effect of controlled renneting and the pre-heat treatment on the coagulation of acidified skim milk. Various extents of controlled renneting including low, medium and high degrees of [kappa]-casein hydrolysis were employed. The pre-heat treatment to skim milk was at 80°C for 30 min. Small deformation dynamic rheological measurement demonstrated that the gelation behavior of the acidified skim milk was changed dramatically after these treatments, and these changes included: onset of gelation occurring at high pH, dynamic elasticity of the gels increasing, and two stages in the gelation development. The scanning electron microscopy (SEM) indicated that the formation of the casein aggregates during the acidification was also different after these treatments. There was no fusion between the contacted casein micelles in the acid skim milk at 30°C, while fusion occurred in the acid skim milk with 20% of [kappa]-casein hydrolysis. After the pre-heat treatment, the casein micelles were found to be connected by the small micellar-bound and soluble denatured whey protein aggregates and formed casein micellar aggregates during the acidification.