Charge‐temperature, modified from bias‐temperature, technique is presented for the investigation of the interface properties of Al‐SiO2‐Si (P) MOS capacitors. By using this technique, the change of interface trap states was found to be related to the treatment of charge‐temperature agin gand consequently the distribution of the mobile charges inside the oxide. An equivalent equation is expressed for the evaluation of interface trap states from the measured C‐V data. In addition, a two‐region model describing the distributions of the mobile charges of the capacitor after various charge‐temperature agings is proposed for the experimental observations. And the fitting results indicate that the effective Debye length due to interface trap states is about 20 Å.