Three-dimensional nonisothermal flow simulations in the kneading disc regions of co-rotating twin screw extruders were performed using a finite element method. The standard Galerkin method and penalty function scheme were applied to the flow field. The streamline-upwind/Petrov-Galerkin scheme was used in the temperature field to reduce numerical oscillation. The simulations were carried out under the operational conditions of The Japan Steel Works TEX30 machine for various rotational speeds. The configuration was ten 2-lobe kneading discs with a 90 degrees stagger angle. Experimental observations were also performed to validate the numerical simulations under the same operational conditions. The pressure in front of the tip in the rotation direction was higher than behind the tip, and the region behind the tip sometimes had a negative value. Since variation of the pressure gradient in the axial direction causes forward;ard and backward flows in the disc gap regions, the disc gap regions play an important role for mixing. The temperature becomes higher with increasing rotation speed due to high viscous dissipation. A high temperature was observed on the disc surface, in the disc gap, and in the intermeshing regions. The numerical results of pressure profiles with the rotation and the temperature in the axial direction were in good agreement with the experimental observations.