A procedure for determining Pu in environmental soil using238Pu as a yield tracer is described. The method involves radiochemical separation and electrodeposition onto a stainless steel disc followed by alpha-spectrometric measurement with a solid-state detector. In order to eliminate error in calculation caused by238Pu contained in original samples, a new calculating method is introduced in this paper. By using this method, the activity of238Pu contained in original samples can be substracted from the total activity of samples, to which the238Pu tracer is added. This procedure has been applied satisfactorily to the determination of Pu in a soil reference sample, which was supplied by the Institute of Metrological Science of China.