lt is of great importance to accurately measure or predict Residence Time Distribution (RTD) in designing extruders and/or in setting up a proper operating condition, because chemical reactions depend significantly on the RTD and temperature when chemical reactions take place during the extrusion process. A previous method to predict the RTD can analytically determnine RTD, Residence Time Distribution Function f(T) and Cumulative Residence Time Distribution Function F(T), based on a simplified 2-dimensional velocity field in an extruder. However, this previous method cannot accurately take into accout the 3-dimensional circulatory flow inside the extruder. The present paper suggests a new method to accurately determine the RTD taking Into account the 3-dimensional circulatory flow and presents a new formula derived to calculate f(T). In order to demonstrate the application of the new method including the circulatory flow effect, RTD, f(T) and F(T) were calculated based on a 3-dimensional velocity field obtained via a quasi-3-dimensional finite element analysis. It was found that the previous method has a tendency to underestimate the RTD due to the neglect of the 3-dimensional circulatory flow in comparison of the new method.