Abstract: The results of this investigation showed that 1) although Pleurotus sajor-caju could grow on Ashby medium, it had no N-fixing activity in various sterilized media. The determination by using the I5N tracing method in 6 replications also made clear that Pleurotus sajor-caju was unable to utilize atmospheric 15N. These facts demonstrated that Pleurotus sajor-caju itself had no nitrogenase activity; 2) after inoculating Pleurotus sajor-caju, the nitrogenase activity war. 19-62 times higher in the medium treated by boiling water than in the medium treated by autoclave. That is because more N-fixing microorganisms were found in the medium treated by boiling water. The pleurotus sajor-caju which was inoculated in the medium sterilized by autoclave failed to fix 15N, whereas the one in the medium treated by boiling water might fix 15N. The N-fixing microorganisms which were isolated from culture medium possessed various levels of nitrogen fixation. These facts illustrated that the increase of nitrogen amount in the cultivation of Pleurotus sajor-caju might be attributed to the associative nitrogen fixation coordinated by different kinds of N-fixing microorganisms in the medium; 3) although the 15N amount fixet in the medium where Pleurotus sajor-caju was grown together with N-fixing microorgainisms was more than ten times higher than that fixed in the medium where only N-fixing microorganisms were grown, it was not always the case that the combining growth of the Pleurotus sajor-caju with N-fixing microorganisms isolated from medium might increase the N-fixing activity. The results received from the experiment showed that some combinations increased N-fixing activity significantly, but some combinations did not.