We demonstrate fine tuning of the resonant wavelength of a nanocavity in a two-dimensional silicon-based photonic crystal slab structure by subnanometer control of the airhole diameter and slab thickness. Theoretical investigation shows that the resonant wavelength depends linearly on the latter two parameters. To experimentally demonstrate the fine tuning of the resonant wavelength, we control these parameters through chemical processes. The resonant-wavelength shift is tuned to 3.25 and 0.36 nm by use of two oxidizing processes. The latter shift, which corresponds to a 0.14 nm thick silicon layer, is considerably smaller than shifts achieved in previous studies. (C) 2009 Optical Society of America