We have developed a method for predicting the three-dimensional flow field in the melt conveying zone in counter-rotating and co-rotating twin-screw extruders. We applied this technique to the full flight screws with thin flight width and open C-shaped channels in both rotating type extruders having the same screw configurations. We compared the details oi velocity and stress fields, the now rates of transportation, and various kinds of leakage flows for both rotating type extruders. Also, we obtained the spatial distribution of tracer particles and residence time distribution using a numerical tracer experiment. The flow rate in the transport direction in the co-rotating twin-screw was larger than that in the counter-rotating twin-screw, and this suggested that. the latter has higher transport performance when the screws have : thin flight width and open C-shaped channels as used in this study, As for the distributive mixing, it was found that the co-rotating twin-screw excels in the area of fluid rearrangement between the two screws and distribution in the rotational direction, while the counter-rotating twin-screw has the desirable characteristic of wide distribution in the axial direction. With regard to dispersive mixing, there was no considerable difference between calculated stress fields in both rotating; type extruders.