Abstract: Reports the determination results of nitrogenase activity of induced cucumber root nudules by ¹⁵N trace method. The root systems bearing induced nodules of cucumber were exposed to a gas mixture containing ¹⁵N₂ for 48h and partial root systems soaked in free-nitrgen culture solution simultaneusly. After exposure the ¹⁵N content in the modulated root systems of cucumber is 0.431 Atom % ¹⁵N by mass spectrometric analysis, whereas in the contrast samples without exposure to ¹⁵N is 0.369 Atom % ¹⁵N. The statiatical t test for the results of ¹⁵N trace experiments is: t=3.15 > t_0.01=2.819, It has been demonstrated that the nitrogenase activity in cucumber nudules is at a remarkable level of 99.9%. The nitrogenase activity in detached nodules of cucumber was also determined by conventional acetylene reduction method. In both methods clear evidences of nitrogenase activity were obtained for the induced nodules of cucumber.