Abstract: The hard oxidation film was proceeded on 6061 aluminum alloy plates with different thickness and the same surface area under the same process parameter conditions. The voltage variation data of different samples were collected dynamically, the oxidation film thickness and hardness of each samples were measured. All measured results were compared and analyzed. Experiment results showed that with greater substrate thickness, the oxidation film grew faster after the dense layer formed, meantime, the final oxide film was thicker and the hardness was higher after the rated time oxidation.Through analysis, it is concluded that under the equal oxidation heat, the larger substrate thickness is, the less increase in internal temperature, which brings less negative influence on the growth of oxidation film, so the hardness and thickness of oxidation film are higher.