The rheological and morphological changes of a low shrink unsaturated polyester resin during thickening and curing were investigated. The behaviour of low profile additive (LPA) could be modified by adding a thermo-breakable functional group to its molecule. The thickening reaction of the modified LPA with MDI was much faster than that of LPA with MgO, and the viscosity was stable for 3 months. Because of the thermo-decomposition of the thermo-breakable functional group, the thickened mixture of LPA-M with MDI lost its viscosity greatly at elevated temperatures. The unthickened LPA and thickened LPA-M/MDI samples showed clear phase separation, large internal surface areas and good shrinkage control. The thickened LPA/MDI samples showed little phase separation, very small internal surface area, and poor shrinkage control. The thickened LPA/MgO samples showed some phase separation and good shrinkage control at high moulding temperature. At low moulding temperatures, both phase separation and shrinkage control were poor.