The hydrolysis rates of seven kinds of polyol ester base oils [POEs] of different branch shape were investigated by using a simple apparatus under a mild acidic condition. Seven polyol ester base oils were made of poly hydric alcohols of two-four valence, normal of branched fatty acid of different carbon number. p-Toluene sulfonic acid was used as an acid catalyst to accelerate the rate of hydrolysis. Partial esters and fatty acid produced by sequential hydrolysis of POEs were identified, and their concentrations were determined by the calibrated-internal stadard method using Gas Chromatography. The rate constants of each step, in sequential hydrolysis, were determined by the least square method from rate equation, and the concentration of each component was compared with one another. It was shown that the rate of hydrolysis of POEs was stronly affected by the molecular structure of fatty acid, a straight chain or a branch chain, and the position of branch. The hydrolysis stability for all the POEs and be reasonably explained by the steric hindrance effect of their molecular structures as water molecule makes an attack on the carbonyl carbon of POEs.