Since composition is an important parameter affecting the dielectric properties in paraelectric SrTiO3layers, composition is determined by Rutherford backscattering spectrometry (RBS) measurement. In this measurement, specifically for achieving precise composition measurement, the RBS spectra of Sr, Ti and O must be separated individually. This spectrum separation can only be attained when thin (800 A[ddot] thick) SrxTiOylayers are deposited on graphite substrate. The measurement is performed for layers deposited at different O2partial pressure ratios and sputtering pressures. This measurement indicates that composition of O, y, in SrxTiOylayer decreases from 3.7 to 2.7 with the decrease of O2partial pressure raito, R(=O2/O2+ Ar) from 1.0 to 0.83. Composition of Sr, x, also changes from 1.1 to 0.6 with this change. With the decrease of sputtering pressure from 10 to 5 mTorr, however, composition, y, is held at 2.7 and only the composition, x, increases from 0.6 to 1.1. This composition measurement is useful for the deposition of optimized dielectric layer employed in the charge storage capacitor.