Terpolymers of methyl methacrylate (MMA), N-phenylmaleimide (PMI) and styrene (St) were synthesized by emulsion copolymerization. The thermal stabilities of terpolymers were studied by a programmed thermogravimetric analysis (TGA) technique. Terpolymers show a considerable increase in decomposition temperature with increasing feed content of PMI and St. The glass transition temperatures (T-g) of copolymers were measured by differential scanning calorimetry (DSC) and torsional braid analysis (TBA). The terpolymers T-g increases markedly with the increasing PMI feed content, while it decreases with increasing St feed content. The rheological behaviors of copolymers were also studied. The terpolymer＇s apparent viscosity in melt decreases with increasing feed contents of PMI and St. The terpolymer＇s now index n increases with the increasing feed content of PMI. The results also show that the difference value between T-gDSC_(_) and T-gTBA. Delta T-g increases with an increase in the terpolymers now index. (M) over bar(w) and (M) over bar(n) of copolymers were also determined by gel permeation chromatography (GPC).