The phosphoric acid method established by J. M. MeCrea in 1950 is generally used to determine the carbon and oxygen isotopic ratios in carbonate minerals. Nevertheless, that method requires the experimenter to select highly pure individual minerals. The method suggested by the authors is based on J. M. McCrea}s technique. Using the different activities of calcite and dolomite at different temperatures and under different reaction conditions, one can collect COz gas step by step from the same sample containing both of the two minerals and then determine carbon and oxygen isotopic ratios separately. Through the experim ent on method conditions and the determining of natural carbonate sa m pies, it is thought that the determination is highly precise and can be used to judge the origin of dolomite. The method is of practical significance. Furthermore, the method has the advantage of determining carbon and oxygen isotopic ratios of the two minerals simultaneously and separately, and rendering it unnecessary to choose and separate individual minerals. The method is easy to use with low production cost and can spare a lot of time and energy.