Among the various methods for developing new polymeric systems, the extrusion process has been increasingly used in the thermoplastics industry in complex applications such as continuous reactors for melt compounding, mixing, and a variety of reaction applications. In any extrusion system, precise testing and analysis is necessary in order to maximize the processing efficiency. The rheological testing measurements on extrudate materials are commonly performed by processors to ensure that their products are meeting the desired qualities and understand the effect of adding various components to their materials. A newly designed on-line rheometer has been developed to continuously monitor the rheological parameters of in-process compounds namely melt flow rate (MFR), intrinsic viscosity (IV), and apparent viscosity. This helps compounders to have a real-time quality control on their products and reduce the failure rates or scraps. The present work aims to describe the design of the on-line rheometer and how it duplicates the test conditions of an off-line melt flow rate tester on an extruder in any compounding or manufacturing process. Moreover, through calculation of activation energy in a specific material and introducing a temperature correlation, it has been investigated how the on-line rheometer considers the temperature dependency of material's MFR and accurately measures this parameter at various operating temperatures.