The effects of different molecular weight distribution of (methylmethacrylate N-phenylmaleimide) copolymers of various molecular weights and their blends on the melt flow index, the storage and loss modulus, and the zero shear viscosity are investigated. Copolymers with higher N-phenylmaleimide contents and definite molecular structure show a typical thermal stability shown by their rheological properties. These studies seem to be useful for first estimations in screening tests and production control.