We propose that Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) can be applied to the characterization of initial oxidation on metal surface, because of its many advantages of quick data collection, parallel detection of wide mass range, high surface sensitivity, high mass accuracy, possibility for hydrogen and its compounds analysis, and so on. In this work, the initial oxidation on Gd surface has been studied through the oxygen exposure experiments, in order to detect surface compounds directly by their fragment ions. Under oxygen exposure of 1.0×10⁻⁵ Pa, positive and negative cluster ions, composed of Gd, O and H, were collected at every cycle of 1 s. Characteristic variation in intensities among the typical ions was observed both in positive and negative spectra, which was supposed to be related to the changes of Gd chemical structure. Based on the results obtained, the initial oxidation process on Gd surface was deduced as follows: at first, suboxide of Gd was generated, followed by hydroxide oxide, and finally Gd hydroxide was covered on the surface. In conclusion, ToF-SIMS can be used for in-situ monitering of initial oxidation as a powerful tool.