A fully transient method for the simulation of the coupled plastic part and mold temperature in the filling stage of Rapid Heat Cycle Molding process was presented. Unlike the constant temperature boundary conditions in conventional injection molding simulation methods, the mold temperature was assumed to be variable in the filling process. The mold temperature in the heating process was first simulated. In the filling process, the flow equations were solved only on the cavity domain, while the energy equations were solved in a coupled manner for the cavity and mold domain at the matrix level. The temperature evolution in a two-dimensional filling process with heated mold was simulated. Numerical results showed that the proposed coupled simulation method provides reliable temperature estimations in mold filling process.