Jatropha curcas (JTC) oil, an inedible vegetable oil, can be a substitute feedstock for traditional food crops in the production of environment friendly and renewable fuel (biodiesel). In the present study, unsupported potassium carbonate was used as a catalyst to provide an understanding of the catalytic activity in the transesterification reaction. Researching the potential and the behavior of potassium carbonate is very important because every biomass ash contains this compound in a significant amount. It can be extracted by using classical extraction or leaching technologies. During the biodiesel production reaction, the formation of soap as a byproduct was also monitored using the FTIR-ATR method. From this study it was observed that the transesterification of JTC oil to JTC biodiesel appeared to be complete within 15 minutes when a 5 wt% (based on the wt. of the oil) potassium carbonate, 6:1 methanol to oil molar ratio, 60°C or a 4 wt% potassium carbonate, 9:1 methanol to oil molar ratio and 60°C reaction temperature were used.