The study of the transition region at the interface of Si-SiO2 by changing the angle of emission in XPS is reported in this paper. The sample are ultra-thin oxide film on the silicon(111) surface formed at low temperature (700 ℃), the thickness of which was less than 50?. The variation of the chemical displacement (δ) and the intensity ratio (I0x/Isi) of the silicon 2p photoelectron peaks come from the oxide film and the single-crystal substrate with the angle of emission is contradictory with the prediction for an ideal interface model. The comparison of the experiment results with the prediction of the random-bonding model shows that there exists a transition region at the interface of Si-SiO2 and the width of which is about 20?, less than the mean escape length of the Si2p photoelectron in SiO2. The same results are given by the experiment of Ar+ ion sputtering profiles.