The voltage-current properties during plasma electrolytic discharge were determined by measuring the current density and cell voltage as functions of processing time and then by mathematical transformation. Correlation between discharge I-V property and the coatings microstructure on aluminum alloy during plasma electrolytic oxidation was determined by comparing the voltage-current properties at different process stages with SEM results of the corresponding coatings. The results show that the uniform passive film corresponds to a I-V property with one critical voltage, and a compound of porous layer and sintered ceramic particles corresponds to a I-V property with two critical voltages. The growth regularity of PEO cermet coatings was also studied. The voltage-current properties during plasma electrolytic discharge were determined by measuring the current density and cell voltage as functions of processing of and the microstructured on aluminum alloys during plasma electrolytic oxidation was determined by comparing the voltage-current properties at different process stages with SEM results of the corresponding coatings. The results show that the uniform passive film corresponds to a IV property with one critical voltage, and a compound of porous layer and sintered ceramic particles correspond to an IV property with two critical voltages. The growth regularity of PEO cermet coatings was also studied.