In this article, the transesterification of waste cooking oil (WCO) to biodiesel fuel (BDF) has been studied using KOH loaded on millimetric gamma-Al2O3 particles in a novel type of two-impinging-jets reactor (TIJR). The effects of various parameters such as KOH loading (wt %), catalyst loading, and methanol-to-oil molar ratio on the BDF yield were studied. The catalyst particles with a KOH loading of 25 wt % at appropriate reaction conditions (i.e., catalyst loading = 4 wt % and methanol-to-oil molar ratio = 15:1) and at 65 degrees C were used in the TIJR. The influences of various operating and design parameters such as jet Reynolds number value, feed flow rate, jet diameter, and the internozzle distance on the performance capability of the TIJR were carefully investigated. As a result of the impinging process, turbulence, and complex trajectory of catalyst particles within the reaction chamber, the BDF yield obtained in the TIJR increased significantly compared to a packed-bed reactor (e.g., 76.35% for TIJR in comparison with 58.01% for the packed-bed reactor after 3 h). This can be attributed to the elimination of external mass-transfer resistance around catalyst particles and remarkably high micromixing of methanol and oil at the impingement zone of the TIJR. Moreover, the mixing that arose from the impinging jets, in TIJR, did not damage the catalyst particles compared to conventional stirrers. Furthermore, the stability of prepared catalyst particles was studied in the TIJR. The results clearly indicate that the characteristics of this novel reactor are favorable for heterogeneous catalysts in transesterification reactions.