We optimize photonic crystal cavities for enhancing the sensitivity to environmental changes by finite-difference time-domain method. For the heterostructure cavity created by local modulation of the air hole radius, the resonance shifts due to refractive index change of the background material are investigated. The shifts can be enhanced by reducing the photonic crystal slab thickness or introducing air holes in the cavity. The sensitivity of the thinner slab with central air holes is 310 nm/RIU (refractive index unit). The heterostructure created in the slotted waveguide of thin PhC slab shows better sensitivity of 512 nm/RIU owing to strong confinement of electric field in the low-index region.