Abstract: The particle diameters of active pharmaceutical ingredient (API) and excipients are important factors to the quality of preparations and have great significances in the reverse engineering to brand products and the consistent evaluation of generic drugs. In this study, a novel method was established for particle size determination to identify the selected component and eliminate other interferential particles by comparing the microscopic images before and after fusion caused by controllable heating. Stearic acid (SA) particles in irregular and spherical shape were selected as a typical excipient to demonstrate the methodology, which were identified from the mixed particles based upon its melting characteristics to detect their particle sizes as well as the size distributions. In the same approach, the morphology and particle size of fenofibrate particles as API in tablets were analyzed. The results illustrated that the particle diameters and particle size distributions of the selected components in the mixture of particles can be detected via the hot-melting characteristics under the prerequisite of proper pretreatment to separate selected components from other particles in microscopic field. In conclusion, this research provides a practical approach for the reverse engineering purpose to brand products and the consistent evaluation of generic drugs.