Coal fly ash-zinc oxide composite (CFA/ZnO) was developed, characterized and applied as a catalyst in the transesterification of used cooking oil (UCO) with methanol to obtain biodiesel. The prepared supported catalyst was characterized by TGA/DTA, gas sorption, TEM-EDX, FTIR and XRD techniques. Taguchi design approach was used for the optimization of biodiesel synthesis from UCO. Amongst the studied process variables, reaction temperature, reaction time and methanol/oil molar ratio most significantly affected the biodiesel yield and fatty acid methyl ester (FAME) content. At the optimum temperature of 140 degrees C, the time of 3 h, methanol/oil molar ratio of 12:1 and catalyst loading of 0.5 wt%, the biodiesel yield was 83.17%. The FAME content of the biodiesel produced was found to be 98.14 wt%. Moreover, the catalyst regeneration and reusability were studied to verify its stability, and it was established that the CFA-ZnO could be reused for up to four successive cycles with negligible loss in performance. (C) 2021 Elsevier Ltd. All rights reserved.