A two-step oxidation method was explored to study the growth mechanism of micro-arc oxidation (MAO) coating. 2A70 aluminum alloy specimens were oxidized in silicious electrolyte in the first step; then the specimens with the as-oxidized MAO coating were transferred into the phosphatic electrolyte for further oxidation in the second step. Distributions of elements in MAO coatings were investigated by energy dispersion X-ray spectrometry (EDS). The results showed that P and Si were layered in the MAO coating. The newly formed coating, P-rich layer, formed in the second step was located under the layer formed in the first step, which increased the coating's thickness. Discharge channels can be found in MAO coating by the distribution of P, which connect substrate and the outside of coating. The inner newly formed P-rich layer increased its thickness as the oxidation time increased. These results give some direct experimental evidences for the inward growth mechanism of MAO coatings and indicate that discharge channels are the main ways to form the new coating which lead to the inward growth of MAO coating. (C) 2013 Elsevier B.V. All rights reserved.