Abstract: After conversion to their p-bromophenacyl esters at room temperature, the epimerit 15(R) and 15 (S) methyl prostaglandin F_2α were completely separated by HPLC on a microparticulate silica gel column (Perkin-Elmer Silica A prepacked) using a mixture of chloroform and ethyl acetate (1:1) as the eluent. The derivatives were detected by UV spectrophotometer at 257nm. The ratio of the epimers in a number of samples were accurately determined by this method. Several samples were quantitatively determined using pure 15 (S) prostagiandin F_2α p-bromophenacyl ester (mp 85～86 C) as the external standard which was prepared in our work. Other prostaglandins including PGF_2a, PGE₁, PGE₂ and ω-ethyl-13-dehydro PGF_2a were also determined by the same method and their relative retention time to 15 (S) PGF_2a p-bromophenacyl ester were given. The method proposed was suitable for accurate determination of 15 (R) and 15 (S) methyl prostaglandin F_2a and their relative contents in samples.