A new method of standard curve analysis associated with X-ray photoelectron spectroscopy (XPS) is presented for measuring the thickness of ultrathin SiO2 layer on Si substrate. In this method, XPS spectra of series SiO2/Si standard samples with different known thicknesses of silicon oxides are firstly recorded, and then the ratios of Si2p peak heights corresponding to SiO2 and Si, viz. R=HSiO2/HSi, are calculated. The known thicknesses of silicon oxides are plotted against the peak height ratios and an XPS standard curve is derived. Under the same experimental conditions, the samples with unknown thicknesses are measured by using XPS technique and then their thicknesses can be obtained from the XPS standard curve. The SiO2/Si standard samples were provided by authoritative lab with the advanced analytical equipments and rich experiences, and the oxide thicknesses were measured by multiple techniques. The present results show that the standard curve, plotted in terms of accuracy of the oxide thickness from the standard samples, can be used for the thickness measurement for ultrathin SiO2 on Si, and this method is valuable in practice owing to the swiftness, convenience and accuracy.