In this study, we introduce a novel index for evaluating the continuous mixing process with pulse-injected bimodal particles. This mixing index is defined by multiplying the Variance among Bimodal Bin Counts (VBBC) by the Residence Time Distribution (RTD) and integrating over time. To apply this method to practical situations, several strategies are introduced, including gridding and selecting time increments for data sampling. Twin Screw Extruders (TSE) are popular and efficient continuous mixers; therefore, as an application example, we examine six cases of hypothetical TSE processes, fitted with different screw arrays of kneading blocks in the mixing zone. Simulations are conducted using Computational Fluid Dynamics (CFD) to obtain data on the exact location of the particles. The mixing index results are compared with the RTD data as a conventional measure of mixing performance, and the best screw array among the different cases is selected based on the proposed mixing index. (C) 2019 Elsevier B.V. All rights reserved.