The non-isothermal flow-field of a mold filling process in resin injection molding with preplaced fiber mats is numerically simulated in the present article. A body-fitted coordinate transformation is applied such that the irregular, moving flow front can be fitted exactly as a boundary, and the error induced from the treatment of boundary conditions is thus reduced. The effects of several parameters. such as types of mats, porosity, mold wall temperature and flow rate of resin at the inlet, on the filling process is deeply investigated.