Branched poly(acrylonitrile-co-vinyl acetate) [P(AN-co-VAc)] was prepared through radical polymerization using new 2-(3-mercaptopropionyloxy) ethyl methacrylate (MPOEM) as a chain transfer monomer (CTM) in sodium thiocyanate (NaSCN) aqueous solution. The development of branching and the changes of molecular weight were analyzed using triple detection size exclusion chromatography (TD-SEC). Below 50% monomer conversion in the presence of MPOEM, the weight average molecular weight (M_w.MALLS) of the copolymer increased with conversion and the molecular weights of the primary chains were much higher. The Zimm branching factor (g′) was lower than one and decreased with increasing conversion, and this result illustrated that the branched chains were formed and the highly branched structures were obtained very fast even at lower monomer conversion. While above 50% monomer conversion in conjunction with complete MPOEM consumption, M_w.MALLS slightly decreased with increasing monomer conversion and almost reached a constant at the end while PDI increased quickly. The g′ slightly increased and then remained constant with increasing conversion, which indicated that the branching degree was invariable in the absence of MPOEM. The zero-shear viscosity and glass transition temperature of branched P(AN-co-VAc)s were lower than those of their linear analogues, which further confirmed the formation of branched P(AN-co-VAc)s. Furthermore, these branched P(AN-co-VAc)s were found to have higher thermal stability than their linear counterparts. These highly branched P(AN-co-VAc)s with lower viscosity and higher thermal stability are amenable for environmentally benign processing with less solvent.