The effect of steam on the oxidative coupling of methane over some basic catalysts was investigated. It has been found that the presence of steam in the reactant stream enhanced the methane conversion, C-2 selectivity as well as the C-2 yield. Another effect of the steam was the enhancement of the ethylene to ethane ratio. When the H2O/CH4 ratio (mel) was in the range of 0-2, the catalytic performance was improved, and the catalyst was also quite stable during 200 h of operation. The beneficial effect of steam on the C-2 yield was more evident when the reactor was enlarged (the catalyst loading was increased from 1 to 30 ml). Two possible reasons account for this effect, one is the dilution effect of steam on the concentrations of O-2 and CO2, another is the uniformity of the temperature distribution along the catalyst bed during the reaction.