Metal‐oxide‐semiconductor (MOS) capacitance–voltage (C–V) characteristics in the accumulation mode have been measured and simulated for polycrystalline Si gate MOS capacitors with various oxide thicknesses (40–200 A˚) onp‐type (100) Si substrates. The discrepancy between experimental data and theoretical prediction by classical MOS theories is clarified by taking quantization effects into account. The experimentally determined ‘‘effective dielectric thicknesses’’ in the semiconductors are found to be in good agreement with the values calculated from quantization effects for MOS capacitors with thinner oxides (<80 A˚). The effective dielectric thicknesses at oxide electric fields of 2–6 MV/cm have been determined to be 2–3 A˚ larger for the quantum mechanical case than for the classical case.