The present invention relates to process for the preparing of optically active ester derivatives and their acid derivatives which are used intensively as important chiral intermediates from racemic .beta.-hydroxybutyl ester derivatives. In more detail, this invention relates to the process for preparing optically active .beta.-hydroxybutyl ester derivatives and their acid derivatives by stereospecific hydrolysis of racemic .beta.-hydroxybutyl ester derivatives using Upases or lipase-producing microorganisms in the aqueous phase or organic phase including aqueous solvent. The method of making optically active ester derivatives and their acid derivatives by hydrolysis of .beta.-hydroxybutyl ester derivatives represented by the general formula 1 in scheme 1 is easier and more economical comparing to the conventional methods and the products have high optical purity. Also separation of ester derivatives from acid derivatives is easy after reaction. Thus this method is a useful process on the industrial scale.