The copolymerization rate of vinyl chloride(VC)/N-phenylmaleimide (PMI) was investigated. The cross termination constant phi was measured to be 8.3 by using nonlinear least square fitting. The value showed that the cross termination was significant. A model of the copolymerization rate of VC/PMI was obtained. Using the calculated modeling parameters, the effects of temperature and initiator concentration on the copolymerization rate were predicted. The predicted values were in good agreement with the experimental data. Acrylonitrile, a third monomer, was selected to reduce the range of copolymer composition of this system, but it further lowered the copolymerization rate.