It is of great importance to understand and predict Residence Time Distribution (RTD) during a singlescrew extrusion process for extruder design and operation optimization purposes. RTD depends on the extruder geometry, operating conditions, and material properties of non-Newtonian fluids. This paper presents the proper dimensionless parameters and RTD for the three-dimensional circulatory flow. These dimensionless parameters were expressed in terms of physical and geometrical parameters such as flow rate, pressure gradient along down channel direction, aspect ratio, helix angle, and power-law index. RTD computations and analysis were based on the finite element method. This dimensionless parameter study is found to be very useful for extruder designers to understand and to predict the RTD systematically.