This paper reports the theoretical and experimental investigations of the strain-induced resonant wavelength shift effect of a modified single-defect 2-dimensional (2D) photonic crystal (PhC) nanocavity resonator. The nanocavity was created by modifying the geometry, i.e., the diameters and shapes, of some specific holes in the triangular-lattice 2D PhC. Structural and optical simulations were performed based on the finite element method (FEM) and finite difference time domain (FDTD), respectively, to theoretically determine the optical characteristics and the strain sensitive effect of the nanocavity. Simulation results showed a linear relationship between strain and the shift of resonant wavelength of the nanocavity. The wavelength shifts due to longitudinal and transverse strains were theoretically determined to be 1.9 and 0.25pm/microstrain, respectively. The PhC nanocavity was also fabricated and the strain sensitive effect was measured. Experimental results confirmed the strain-induced resonant wavelength shift effect in the nanocavity. The resonant peak of the nanocavity was shifted about 100 pm to the longer wavelength when the nanocavity was stretched with a tensile strain of 300microstrain along the light-transmission direction. These results show a potential of using a PhC cavity to detect the strain by monitoring its resonant wavelength shift.