A numerical study of three-dimensional fluid flow and heat transfer in the metering section of a single screw extruder has been performed. The screw channel was simplified by unwinding the channel and the coordinate system was fixed on the screw root in the present model. The pressure distribution over the inlet region was to be extrapolated from the solution of interior pressure. The prescribed mass flow rate was imposed on the outlet of the unwinding screw channel. The three-dimensional velocity fields, temperature, viscosity, and pressure profiles in the screw channel were calculated by using the commercial code, STAR-CD, based on the finite volume method Some of the results of the present full 3D model were compared with the results from the marching-type 3D model. It is shown that the present work can capture the back flow by the pressure gradient near the screw root in the cases of small mass flow rates. There exist significant discrepancies in temperature between the marching-type 3D model and the present 3D model. The present work well describes the effects of heating-up through the circulation flow and viscous dissipation. The cross-section contours of velocities, temperature, viscosities and pressures along the down-channel direction are also shown in this study.