The effects of cylinder site displacements(site randomness) and cylinder radius variations(size randomness) on photonic band gaps of the photonic crystals,which consist of a graphite lattice of dielectric cylinders,have been discussed by the Order-N method with a supercell technique. The numerical result shows that the variation of the photonic band gaps for the E polarization mode is far more sensitive to disorders with a size randomness than with a site randomness. Furthermore,the influence of the disorder on light extraction of a GaN light emitting diodes model,which has a disordered photonic crystal slab,has been investigated by three-dimensional finite-difference time-domain method. The numerical result shows that the influence of the disorder is small on light extraction of this light-emitting diodes model,and the influence is random.