We present in-depth discussion of the design and optimization of a nanomechanical sensor using a silicon cantilever comprising a two-dimensional photonic crystal (PC) nanocavity resonator arranged in a U-shaped silicon PC waveguide. For example, the minimum detectable strain, vertical deflection at the cantilever end, and force load are observed as 0.0133%, $0.37\mathrm{\mu m}$ , and $0.0625\mathrm{\mu N}$ , respectively, for a $30\mathrm{\mu m}$ long and $15\mathrm{\mu m}$ wide cantilever. In the graph of strain versus resonant wavelength shift, a rather linear relationship is observed for various data derived from different cantilevers. Both the resonant wavelength and the resonant wavelength shift of cantilevers under deformation or force loads are mainly a function of defect length change. Results point out that all these mechanical parameters are mainly dependent on the defect length of the PC nanocavity resonator. This new PC cantilever sensor shows promising linear characteristics as an optical nanomechanical sensor.