Yang, Z.Q. Lu, X.Y. Tan, W.W. Yang, D.Y. Yang, Y. Zhao, J. JACoW Geneva, Switzerland Fundamental understanding of the nitrogen doping mechanism has been carried out extensively, but yet remains unclear. We studied the nitrogen doping effect through a series of experiments on niobium samples at both room and low temperature circumstances to seek the physical explanation based on the proposed nitrogen-hydrogen interaction. The time of flight secondary ion mass spectrometry (TOF-SIMS) revealed that the diffused nitrogen plays a key role in the nitrogen doping effect. X-ray photoelectron spectroscopy (XPS) measurements showed the chemical distribution of it. The effects of different concentrations of diffused nitrogen on the samples' magnetic property were studied by the magnetic property measurement system (MPMS). Direct observation of the surface topography of both doped and undoped samples by using scanning electron microscope (SEM) at liquid nitrogen temperature has proved that the diffused nitrogen can prevent or retard the hydrides formation to varying degrees with different amounts of material removal. A possible mechanism of nitrogen-doping effect is proposed based on our study.