A thermally stimulated surface potential (TSSP) method has been developed to estimate the parameters of the insulator‐semiconductor interface in MIS structures. As TSSP is measured in the open‐circuit condition, charge on the gate electrode and voltage applied across the insulating layer are both constant in the measurement. For the above reasons, the analysis of TSSP is simplified, and theoretical TSSP curves can be calculated under non‐steady‐state and steady‐state conditions. The samples used for our experiments are Al‐SiO2‐Si MIS structures. To estimate the energy distribution of interface states, the temperature‐vs‐potential characteristic is used, particularly the point where the interface changes from non‐steady‐state to steady state. The characteristic can be clearly and easily obtained by a convenient method devised in the present work. The total amount of interface states, their energy distribution, and their lifetime are estimated in a wide energy range from the characteristic by simple calculation. Further, the relaxation mechanisms of the interface are discussed for samples having substrates of bothp‐ andn‐type Si.