Abstract: Ni/MgO is a kind of effective and stable catalyst for ethanol steam reforming reaction. The effect of nickel loading on magnesia for ethanol steam reforming reaction was investigated in the present paper. It is indicated, under the condition of 101.3kPa, that the activity of the catalyst increases with the increase of nickel loading on magnesia. As to the selectivity to hydrogen, there is an optimum loading of nickel (10%Ni/MgO). The hydrogen selectivity order of catalysts with different nickel loading is 10Ni/MgO>15Ni/MgO>12.5Ni/MgO>7.5Ni/MgO≈5Ni/MgO. The thermoanalysis results of the catalysts show that interactions between the precursors of nickel and carrier change with the nickel loading during the calcination treatment. XRD and TPR analysis prove that the crystal structure and reducibility of the catalysts are affected by nickel loading. Two kinds of interactions between the nickel precursor and carrier are proposed for 10Ni/MgO during the calcination. The oxide state crystal structure and reducibility of 10%Ni/MgO are also different from others. It implies that the selectivity of the catalyst is affected by not only the active phase Ni but also the surroundings of the active phase Ni.