We present the design of a double-slot photonic crystal cavity as an optomechanical device which contains a nanomechanical resonator with an effective mass as small as 6.91 fg. The optical Q-factor is optimized to 2 × 10(5). Using phononic crystals, the mechanical vibration is confined in a small volume to form a mechanical mode of 4 GHz with a high mechanical Q-factor and a femtogram effective mass. The localized mechanical mode overlaps with the optical field and strengthens the optomechanical coupling with a vacuum optomechanical coupling rate g0/2π exceeding 600 kHz. Considering fabrication imperfections, structures with deviation from ideal design are studied. The symmetry breakage of the structures and the displacement fields makes the mechanical effective masses reduced and close to 4 fg. The devices can be used in ultrasensitive sensing of mass, force and displacement.