Temperature-programmed reduction (TPR) and continuous stepwise reduction-methanation (CSRM) have been used to identify and separate the active phases of Ni/Al2O3 catalyst. It is found that there are four hydrogen consumption peaks representing four nickel species on Al2O3; the peak temperatures for three of them are below 673K when the reduction temperature does not exceed to 973K. For the methanation of both CO and CO2, three distinct active phases are discovered, the activation temperatures of which coincide with the peak temperatures of the first three TPR peaks, respectively. This indicates that different oxidized nickel species exist on Al2O3 prior to reduction and form different active phases for the methanation of both CO and CO2 after reduction. The methanation activities on each phase for both CO and CO2 in terms of CH4 yield or initial methanation temperature and turnover number differ significantly. Each phase also exhibits different activity for CO and CO2 methanation. It is suggested that the mechanism of methanation for CO or CO2 on different phases may vary, and that the mechanism for CO and CO2 on the same phase may also vary. CSRM is convenient for separating different active phases for the methanation of both CO and CO2 on Ni/Al2O3 catalyst.