To solve the problems generally encountered during the plasma electrolytic oxidation (PEO) of Al alloys with high Si content, a pretreatment of chemical etching was applied before the process. The influence of such pretreatment was studied by SEM, EDS and XRD. The pretreatment presents a significant effect on positive voltage at the beginning stage of PEO, leading to higher voltage over the whole process. The difference between the positive voltages of non-etched and etched specimens decreases gradually with the increase of processing time. The pretreatment exhibits much less influence on the negative voltage. For the sample with surface pretreatment, the average growth rate of PEO coating is increased from 0.50 to 0.84 mu m.min(-1) and the energy consumption is decreased from 6.30 to 4.36 kW.h.mu m(-1).m(-2). At the same time, both mullite and amorphous SiO2 contents are decreased in the coating. (C) 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.