In this study, the effect of agitation rate on the aggregation morphology of the suspension poly(vinyl chloride) (PVC) grains, especially their primary particles, at different conversions was investigated. The results of scanning electron microscopy and laser particle size analysis indicated that the size and particle size distribution (PSD) of the final resin grains were mainly affected by the agitation rate, which were obtained before the monomer conversion reached 27%. A high agitation rate was beneficial to obtain PVC resin with small grains and narrow PSD. However, higher agitation rate after 27% conversion would lead to more aggregation and fusion of the primary particles and make the primary particles larger. The resins produced by a variable speed process (from 600 rpm to 300 rpm when the grains were basically formed, i.e., 27% conversion) displayed better aggregation morphology and processability compared to those produced by the simpler high or low agitation rate method. Moreover, the relationship between the aggregation morphology and the macroscopic properties of the PVC resin was established.