The deep level in both Si/SiO2 interface and its neighbourhood and Si/SiO2 interface states have been studied systematically with Deep Level Transient Spectroscopy (DLTS). Experimental results show that a dominant deep level, H (0.494), exits in both Si/SiO2 interface and its neighborhood in MOS structure formed by thermal oxidation. The deep level possesses some interesting properties, for example, its DLTS peak height depends strongly on temperature, and when the gate voltage reduces the interval between. Fermi-level and Si valence band to a value less than that between the deep level and Si valence band, its sharp DLTS peak is still observable. The hole capture cross section of the interface states at Si/SiO2 interface has been found to depend on temperature, measured with a new method. The energy distribution of the interface states measured with DLTS contradicts to the distribution obtained by quasistatic C-V technique. A new physical model of Si/SiO2 interface was proposed, with this model the above stated experimental results can be successfully expounded.