The reliability of the FE method and the CV method for determining the localized state distribution N(E) in GD a-Si are evaluated by computer simulation. It is shown that calculation by the CV method is almost exact and the FE method is less reliable. But the FE method may be more reliable than the CV method about the accuracy of measured values. The new calculation method combining both methods and the iteration method are suggested for obtaining almost true distribution N(E). And the effect of surface states on calculation of N(E) is evaluated; the allowable surface state density is shown to be below 1011cm-2eV-1. The effect of the Fermi-Dirac distribution function F(E) at room temperature is estimated. The results indicate that correction by room temperature F(E) is necessary.