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A quantum-mechanical (QM) model is presented for accumulation gate capacitance of MOS structures with high-κ gate dielectrics. The model incorporates effects due to penetration of wave functions of accumulation carriers into the gate dielectric. Excellent agreement is obtained between simulation and experimental C-V data. It is found that the slope of the C-V curves in weak and moderate accumulation as well as gate capacitance in strong accumulation varies from one dielectric material to another. Inclusion of penetration effect is essential to accurately describe this behavior. The physically based calculation shows that the relationship between the accumulation semiconductor capacitance and Si surface potential may be approximated by a linear function in moderate accumulation. Using this relationship, a simple technique to extract dielectric capacitance for high-κ gate dielectrics is proposed. The accuracy of the technique is verified by successfully applying the method to a number of different simulated and experimental C-V characteristics. The proposed technique is also compared with another method available in the literature. The improvements made in the proposed technique by properly incorporating QM and other physical effects are clearly demonstrated.