In the present work, a digital image processing (DIP) method was developed to measure the residence time distribution (RTD) in a plasticating extruder. The DIP method is off-line, but it consumes less time and yields more data than the traditional off-line ones, and it avoids the expensive probes and data acquisition systems that are necessary in the on-line and in-line measurements. The correctness of the DIP method was validated by a comparative experiment using silica powder as the tracer. A locally made Buss Kneader equipped with a gear pump system was used herein. The effects of the operating conditions on the RTD in this equipment were also studied. Furthermore, the relationship between the mean residence time and the operating conditions was analyzed.