The in-line Residence Time Distribution (RTD) measurement during the extrusion process makes possible the analysis of the extruder performance and the polymer melt flow behavior. It is also possible to evaluate the involved processing variables and to optimize the extrusion process in order to obtain an effective shear rate and hence an improved transport capability of the polymer along the extruder. In this work the effect of the processing variables (feeding rate and screw speed) and the polymer melt flow index (MFI) on the RTD in a modular co-rotational twin screw extruder was investigated. The results showed that the feeding rate had more influence on the DTR than the screw speed. The RTD curves were affected by the change in the polymer MFI.