Jatropha oil was extracted from seeds of Jatropha Curcas cultivated in Nakhon Ratchasima, Thailand and analyzed by various methods to determine its composition and properties. It was used as a feedstock to produce biodiesel via transesterification over heterogeneous catalysts consisting of potassium supported on NaY (K/NaY) with potassium loading of 4, 8 and 12 wt%. The catalysts were prepared by impregnation a buffer solution of potassium on NaY to prevent collapses of the zeolite structure. The transesterification was primarily monitored by thin layer chromatography and products from the most suitable condition were further analyzed by gas chromatography. The catalyst with 12 wt% of potassium loading provided the optimum biodiesel yield of 73.4% under the reaction time of 3 h, the reaction temperature of 65 ℃ and methanol to oil molar ratio of 16:1.