Terpolymers of N-phenylmaleimide (PMI), methyl methacrylate (MMA), and acrylonitrile (AN) were synthesized by emulsion polymerization. The thermal properties of the terpolymers, at different PMI and AN feed contents, were investigated by TEA, TGA, and the Vicat softening point test. The results show that the glass transition temperature (T-g) and decomposition temperature of the terpolymers increase with increasing PMI feed content. Furthermore, the Vicat softening points of the terpolymers increase with the PMI feed content. The (M) over bar (w) and (M) over bar (n) of the terpolymers were also determined by GPC. The results show that the (M) over bar (w) and (M) over bar (n) of the terpolymers have a maximum value in the range of 0-30% PMI feed content and decrease with an increasing AN feed content. The mechanical properties (tensile strength and impact strength) of the terpolymers decrease with an increasing PMI feed content and increase with an increasing AN feed content.