This work aims at simulation by particle tracking the local residence time distributions (RTDs) of a co-rotating twin-screw extruder using computational fluid dynamics. Simulated results follow reasonably well the trend of experimental results obtained by an in-line measuring instrument for different screw configurations and feed rates. To analyze the distributive mixing performance and overall efficiency of different types of kneading discs (KDs), mixing parameters such as area stretch ratio, instantaneous efficiency, and time-average efficiency are calculated. Among KDs with stagger angles 45 degrees, 60 degrees, and 90 degrees, the 90/10/64 with disc gaps is most efficient in terms of distributive mixing. The effects of the disc width and disc gap on the local RTD and distributive mixing are also discussed. This provides a numerical tool for assessing point-by-point information on the local RTD, flow, and mixing along the screw extruder. POLYM. ENG. SCI., 49:1772-1783, 2009. (C) 2009 Society of Plastics Engineers