When predicting the variation of pore structure during CO2 gasification of coal chars using the random pore model (RPM), it is impossible to calculate exactly the psi parameter from the pore characteristics, which were obtained by means of N-2 adsorption, such as BET surface area (denoted as N-2 pore characteristics), of the char prior to gasification. The values of psi, which were calculated from the pore characteristics of chars at various carbon conversions, should be fundamentally constant, unaffected by the conversion of the char. However, this investigation exhibited a drastic decrease of psi at the initial stage of the gasification reaction. This phenomenon is the result of a significant increase of N2 pore characteristics, of which the starting chars are extremely small. This increase might be explained by the widening of submicropores which are undetectable through the N-2 adsorption method or by the reopening of closed pores inaccessible even to helium molecules, followed by the formation of new micropores exceeding the detection limit of N-2. Consequently, this study introduced the volume of submicropores and closed pores into the psi equation as correction terms. The value of psi at the reaction starting point was close to that at the intermediate stage of reaction, indicating that the accuracy for psi estimation was elevated and that the submicropores and closed pores should be taken into account when using RPM.