The antireflective characteristics of Si nanocone (NC) arrays were estimated using a theory devised for an inhomogeneous antireflection layer, and further verified by the Fourier modal method (FMM). Considering a better impedance matching from air to Si, a minimum depth of 400 nm is essentially required. Although Si NC arrays have usually been suggested to be at a base diameter of ∼300 nm for infinitely thick Si wafers, as wafers become thinner than 50 μm, the optimal base diameter of the NCs is suggested to be ∼500 nm so as to excite more resonant modes. Our simulation work indicates that geometrical parameters such as the top diameter and filling ratio of the NCs are much more sensitive in terms of optimizing the optical performance on ultrathin (∼5 μm) wafers, suggesting the need for strict control of the surface morphology in the nanostructure fabrication process.