The influence of chain length and the extent of the saturation on the esterification activity of long chain carboxylic acids were studied on Nb2O5 catalyst. A series of individual fatty acids (caprylic, capric, lauric, myristic, palmitic, and steric acids) and their mixtures present in vegetable oils, namely, coconut oil, ground nut oil, olive oil, and soybean oil, were selected for esterification with methanol at different reaction temperatures, The esterification reactivity was found to be inversely proportional to alkyl chain length of the acid. The trend was discussed in terms of steric and inductive effects. The kinetic parameters estimated revealed a mild dependency of the activity on the extent of unsaturation.