Abstract: According to JJF 1059-2012 and GB/T 6462-2005,the thickness of the piston hard anodized layer was observed and measured by optical microscope and metallographical analysis software.From the aspects of test repeatability,sample mounting perpendicularity and optical microscope calibration,the influence on thickness measurement uncertainty of piston hard anodized payer was analyzed and illustrated.Each uncertainty factor was evaluated,calculated and combined,and finally the expanded uncertainty was give and the measurement uncertainty was reported.The result show that the thickness of piston hard anodized layer was 10.13 μm,and the expanded uncertainty U was 1.24 μm when the coverage factor k was 2.