Fatty acids (FAs) are produced from the hydrolysis of an oil feedstock. Oil hydrolysis is often carried out in the presence of subcritical water (SCW). SCW has low dielectric constant and high ionic product that serves good miscibility with non-polar substances and greatly influences acidbase catalysis. This study focused in the evaluation of acid catalysis mechanism in the hydrolysis of used cooking oil. Three different types of acid investigated were FA (autocatalysis), H2SO4, and in situ Cu soap formed from the reaction between synthetic Cu-containing wastewater and the FAs. Hydrolysis reaction was performed in a batch autoclave reactor at 180- 250°C for 2 h with a fixed oil to water molar ratio of 1:30. The results indicate that lower polarity and higher ionic product of water dramatically enhanced the catalysis effect of H2SO4 and Cu soap by more proton production and the promotion of Cu soap formation. At the same initial pH of 5.0, H2SO4 and Cu soap catalysis demonstrated relatively same behavior in SCW. Weaker effect of SCW on the autocatalysis mechanism was attributed to the less dissociation of FAs at elevated temperature, which resulted in fewer protons available for catalysis.