The nuclear scattering of 3 MeV protons on oxygen is applied to determine the oxygen stoichiometry in high Tc superconducting (HTS) thin films. Their oxide substrates are used as reference standards. This is a nondestructive, absolute method, which is as simple as the traditional Rutherford Backscattering Spectrometry (RBS). However, the relative sensitivity to oxygen has been enhanced. To obtain the stoichiometry, one only needs the data on atomic stopping powers and the chemical composition of the substates.When the measurements on the thin film and on the substate are carried out simultaneously, the inaccuracy in charge integration can be avoided and the accuracy of the oxygen stoichiometry can be further improved.