Toluene as a model tar compound was used to study the effect of atmosphere on carbon deposition during biomass tar reforming. A novel Ni-loaded on lignite char (Ni/LC) and commercial Ni/Al2O3 was employed for stability evaluation in a thermogravimetric analyzer. The mechanism of coke formation under N-2, H-2 and steam with different steam/carbon (S/C) ratio were investigated during 5 h test. Nickel particle growth is the main reasons responsible for the deactivation of Ni-based catalysts for tar reforming. Steam remarkably suppressed the carbon deposition on Ni/Al2O3, especially in a high S/C ratio. Ni/Al2O3 exhibited high activity and stability for 5 h operation in S/C ratio of 2. H-2 significantly promoted the carbon deposition on Ni/Al2O3 and caused the catalyst deactivation within 0.5 h. Ni/LC exhibited great resistance to coke deposition under inert and H-2 reforming of toluene. The catalysts before and after catalytic cracking were characterized by X-ray diffraction and transmission electron microscopy to investigate the behavior of carbon deposition. Except for H-2 reforming, an obvious change of the Ni crystallite size (NCS) can be found after reforming for 5 h under all conditions used in this study. The NCS in Ni/LC was significantly increased with increasing time and S/C ratio, which should be partly responsible for the deactivation of the Ni/LC.