A dilatometry and morphological study of the effect of the various thermoplastic additives on shrinkage control of unsaturated polyester (UP) resins cured at low temperatures is presented in this article. It was found that shrinkage control is closely related to the morphological changes during curing. There are two major transitions in the relationship of shrinkage and additive concentration that can be explained by the sample morphology. The additive starts to be effective on shrinkage control at the first transition, when a co-continuous phase structure exists in the sample at a certain additive concentration. The shrinkage control effect vanishes at the second transition, when severe particle fusion is observed in the sample because of too high an additive concentration. The effective additive concentration range does not change much for various thermoplastics, but the two transitions occur at different concentrations for each thermoplastic.