We have analyzed a hybrid photonic-plasmonic crystal nanocavity consisting of a silicon grating nanowire adjacent to a metal surface with a gain gap between. The hybrid plasmonic cavity modes are highly confined in the gap due to the coupling of photonic crystal cavity modes and surface plasmonic gap modes. Using the finite-element method, we numerically solve guided modes of the hybrid plasmonic waveguide at a wavelength of 1.55 μm. The modal characteristics such as waveguide confinement factors and modal losses of the fundamental hybrid plasmonic modes are explored as a function of the groove depth at various gap heights. After that, we show the band structure of the hybrid crystal modes, corresponding to a wide band gap of 17.8 THz. To effectively trap the optical modes, we introduce a single defect into the hybrid crystal. At a deep sub-wavelength defect length as small as 180 nm, the resonant mode exhibits a high quality factor of 566.5 and an ultrasmall mode volume of 0.00186 (λ/n) 3 at the resonance wavelength of 1.55 μm. In comparison to the conventional photonic crystal nanowire cavity in the absence of metal surface, the figure of merit Q/Vm is enormously enhanced around 15 times. The proposed nanocavities open up the opportunities for various applications with strong light-matter interaction such as nanolasers and biosensors.