It is typically difficult to measure viscosity of a fast polymerizing molding compound using a rheometer. So, spiral mold filling experimental data were used to obtain viscosity functions of a silica filled epoxy-novolac molding compound. The functions could describe the effect of temperature and conversion on viscosity change during cure. With the flow rate data obtained from the mold filling experiments, parameters of the viscosity functions were determined through regression of a simulation model developed in this study. The reaction kinetic equation considering an autocatalytic reaction mechanism was used for the molding compound in the simulation model. The viscosity function of the molding compound determined from the rheometer data and the reaction kinetic data measured at a relatively slow reacting condition was compared with simulation results. The viscosity function determined through nonlinear regression of the spiral mold filling simulation model showed a good correlation with the viscosity function obtained from the rheometric study.