Mineralization and immobilization of soil nitrogen were studied by incubation methods with labelled nitrogen. Results showed that deoamposition of organic nitrogen in soil was not accelerated by the mineral nitrogen added;however, the mineral nitrogen added in soil accelerated the biological immobilization of the nitrogen added, and, at the wane time, lowered the immobilization of soil nitrogen. Therefore, there was no dietinct fluctuation of the total immobilization nitrogen in soil in the whole period of in-cwbation. It seems that the increase of the net mineralized nitrogen in soil induced by mineral nitrogen added, which is generally Known as the priming effect, is mainly because part of soil nitrogen to be immobilized is replaced by mineral nitrogen added in soil.