The static dielectric constant of isolated silicon nanocrystals (nc-Si) embedded in a SiO2 thin film synthesized by Si+ implantation has been determined from capacitance measurement based on the Maxwell–Garnett effective medium approximation and the stopping and range of ions in matter simulation. For the nc-Si with a mean size of ∼ 4.5 nm, the dielectric constant so determined is 9.8, being consistent with a theoretical prediction. This value is significantly lower than the static dielectric constant (11.9) of bulk crystalline Si, indicating the significance of nc-Si size effect. The information of nc-Si dielectric constant is not only important to the fundamental physics but also useful to the design and modeling of nc-Si-based memory devices.