Abstract: Quantum capacitance is an increasingly important parameter in semiconducting nanomaterials and devices. By measuring and extracting the quantum capacitance of graphene, we can infer important physical properties of graphene, and it is also a useful guide for the vertical scaling of graphene field-effect transistors. In this work, a new simple method has been used to fabricate ultrathin, high-quality Y2O3 gate dielectric on graphene with an equivalent oxide thickness as low as 1.5 nm. By changing the thickness, the quantum capacitance has been accurately measured and extracted, with results that agree well with the theoretical calculation when far away from the Dirac point. Furthermore, a microscopic model of the quantum capacitance based on potential fluctuations has been developed which can quantitatively describe the measured values near the Dirac point through use of the single parameter-potential fluctuation. Our model fits the experimental results excellently throughout the whole energy range. In addition, we have explored the performance limits and potential advantages of graphene transistors compared with Ⅲ-Ⅴ field-effect transistors from the perspective of quantum capacitance