Effect of molecular weight of a thermoplastic copolymer on final morphology of polymerization induced phase separation of trimethylolpropane triacrylate/copolymer blends have been studied. Two acrylate copolymers, ACHM and ACLM, mainly based on methyl methacrylate, butyl acrylate and styrene were synthesized with high and low molecular weight, respectively. Final morphology of TMPTA/ACHM and TMPTA/ACLM blends were investigated by optical microscopic observation of ensemble of surface and bulk phase separation status points of view. Image analysis were done to provide related characteristic length scales.Polymerization induced phase separation of TMPTA/ACHM and TMPTA/ACLM resulted in different final morphologies with different characteristic length scales, which was attributed to the kinetic effect of the copolymer chains mobility during two-step phase separation induced during the thermal history and UV-irradiation periods.