An important feature of free radical crosslinking copolymerization of unsaturated polyester resin (UP) - styrene is the formation of a heterogeneous structure through intra-molecular reactions and phase separation. Such structure formation affects not only the cure behavior, but also the rheological changes. The chemorheology and the effect of shear on the structure formation, as well as the rheological changes, were investigated using rheometry, electron spin resonance spectroscopy, and dynamic Light scattering goniometry. Based on experimental results, a conceptual model and a simple mathematical formulation are proposed to explain the shear effect on resin viscosity changes. Predictions obtained from the mathematical model are compared with experimental results.