The 2A12 aluminum alloy surface is treated by plasma electrochemical oxidation (PEO) in alkaline solution. X-ray Diffraction (XRD) is used to analyze the phases of PEO coatings. Scanning Electron Micrograph (SEM) is used to investigate the compactness of PEO coatings. High resistance meter and voltage resistance meter are used to measured Electric resistance and breakdown field strength of PEO coatings respectively. The results show that the PEO coating is composed of α-Al₂O₃, γ-Al₂O3 , etc, and they are all well-insulated substance which is profitable to insulation of 2A12 aluminum alloy. When the thickness of PEO is more than 60μm, porosity in the PEO coating is estimated not more than 15%. Electric resistivity of PEO coatings can reach as high as 10¹³ Ω·cm, and breakdown field strength of the PEO coatings can reach 72 V / μm.