Abstract: The effects of temperature and time on the nature of carbon deposition over Ni / Al2O3 catalyst were investigated using an on - line quadruple mass spectrometer method. The experimental results suggested that the initial carbon species，viz. NiXC，produced by CH4 decomposition over reduced Ni / Al2O3，trends to diffuse into the bulk at lower temperature（600700℃）and to transform into more inactive carbon species at higher temperature（700800℃）and longer duration. The catalyst deactivates quickly for CH4 decomposition at higher temperature due to the blocking of active sites by immobile carbon species.