The main objective of the present work was to introduce an in-line method for measuring the residence time distribution (RTD) of twin screw extruders on the basis of information obtained from the melt free radical modification of polyethylene in a modular intermeshing co-rotating twin screw extruder The trend of increasing rate of the extruder total torque resulted from replacing the neat polyethylene feed by a mixture of polyethylene and reactants which are chemically capable of creating chainbranching and/or crosslinking reactions was considered as the main information source for evaluating the RTD of the extruder The RTD results were compared with those obtained from the tracer pulse input method, and good agreement was found. It was also demonstrated that, the present in-line RTD measuring technique has reliability of following the effect of feeding rate, screw speed and reactant concentration on the RTD of the twin screw extruders. It was found that, increasing the feeding rate results in decreasing the minimum residence time, and results in a narrower RTD. At constant feeding rate increasing the screw speed decreased the minimum residence time and broadened the RTD. The results also showed that, reactant concentration and variation of the viscosity along the screw have not appreciable effect on the twin screw extruders RTD.