Abstract: The residence time and its distribution during the bulk p olymerization of styrene in a twin screw extruder were investigated. It is found that the residence time of the reagents in the twin screw extruder covers t he r ange from 1 min to 4 min. Furthermore, the residence time is grea tly affected by the rotation speed, the feed speed and average molecular weight of the polymer. It is also found that the polymerization mainly generates at the zone on the screw about 60 cm length from initial position and fully completes at th at about 85 cm length. Based on dimensional analysis, a model o f the residence time has been established, which is confirmed by the results of realistic measurements.