Use of low-cost raw materials containing high amounts of free fatty acids for the production of biodiesel is a valuable alternative. The production of biodiesel from these kinds of raw materials involves a two-step process. The first step requires esterification of free fatty acids by acid catalysis, followed by base catalyzed transesterification. Karanja is a non-edible oil, with high free fatty acid content, found in abundance in India. Phosphoric acid modified H-Y zeolite was used as a solid acid catalyst for free fatty acid esterification in Karanja oil. The principal parameters of the esterification reaction have been investigated and optimized via the classical method. Kinetic measurements revealed that the esterification followed pseudo first order kinetics.