We report a high-Q design for a semiconductor-based two-dimensional zero-cell photonic crystal (PhC) nanocavity with a small mode volume. The optimization of displacements of hexagonal-lattice air holes in the Gamma-M direction, in addition to the Gamma-K direction, resulted in a cavity quality factor Q of 2.8 x 10(5) sustaining the small modal volume of 0.23(lambda(0)/n)(3). The momentum space consideration of out-of-plane radiation loss showed that the optimization of air hole displacements in both the in-plane x and y directions reduced FT components in the leaky region along the k(x) and k(y) axes, respectively. This high-Q cavity design is applicable to Si and GaAs semiconductor materials. (c) 2010 Optical Society of America