An important feature in the cure of unsaturated polyester (UP)/styrene/thermoplastics system is the formation of a two-phase structure. Its final morphology is primarily determined by the phase separation process and the gelation resulting from the polymerization. In this study, the phase separation process during the cure of UP resins with thermoplastic additives was investigated by optical microscopy. It was found that depending on the system miscibility and reaction kinetics, the formation of sample structure follows the same route, but may end at different stages with different types of structure. Two key factors-the volume fraction of the thermoplastic-rich phase and the phase separation period i.e. the time period between the onset of phase separation and the gelation-determine the final structure of the sample. Based on experimental results, a shrinkage control mechanism at low temperature cure was proposed.