Abstract: The effect of process parameters on the electric insulance of micro arc oxidated ceramic coat on aluminium plate is studied. The results show that the insulance of micro arc oxidated ceramic coat increase with the rising of current density and the prolonging of oxidation time respectively within some critical values, which is attributed to the increase of thickness and density of the ceramic coat. When the current density and oxidation time go beyond the critical values, the thickness and density of the coat decrease and so does the insulance.