A high free fatty acid (FFA) containing Jatropha curcas L. oil (JCO) was used for biodiesel production using Musa balbisiana Colla underground stem (MBCUS) ash catalyst, in a high pressure-high temperature (HT) reactor. The composition of the MBCUS catalyst was ascertained using XRF analysis and found that the ash obtained at 550 A degrees C contain about 36 % silica (SiO2), 25 % potassium oxide (K2O), 10 % each of lime (CaO) and magnesia (MgO) and 4 % each of phosphorous pentoxide (P2O5) and alumina (Al2O3) as the major component for catalytic activation. The catalyst was characterized by SEM-EDX image analysis and found that the composition of the catalyst is versatile and consist of several alkali and alkaline earth metal chlorides, oxides, carbonate and silica. The BET surface area was found to be about 39 m(2)/g. The catalyst was further calcined in a TGA and observed that a maximum of 10 % losses were possible at 991 A degrees C. The catalyst structure was further characterized from TEM images and found that the structural dimensions are identical (less than 100 nm length) and brick like (70 nm x 20 nm x 15 nm). The catalyst was found very effective during transesterification under HT (275 A degrees C) and internal pressure (4.2 MPa) and 98.0 % fatty acid methyl ester (FAME) could be obtained from JCO with an initial acid value (AV) 18.4 mg KOH/g. There was a drastic reduction of FFA (AV 3.4 mg KOH/g) in the FAME obtained. The reduction of FFA may be attributed to the HT and high internal vapour pressure of the reaction mixture that resulted conversion of FFA to FAME and water. The fuel properties of the FAME were analysed as per ASTM and EN-standards.