Owing to highly nonlinear and anomalous dispersion, light propagation in Photonic Crystals(PCs) shows unusual characteristics which are very different from conventional cases. In order to interpret the abnormal phenomena, we calculate anisotropic dispersion surface derived from nonlinear photonic band structure. Using this equifrequency surface in two dimensional k-space, we can explain various odd phenomena of light in PCs of triangular and square lattice. In particular, in PC of r=0.4a, refractive index=3.24, square lattice holes, the difference of refractive indices between air and PCs and four-fold symmetry of square lattice make the band gap look to be larger effectively compared to that shown in band diagram. Physically this is originated from the fact that the wavefront of light in air is not matched to that of light in PCs at interface. Moreover when the PC`s refractive index is changed by external fields we describe change of dispersion surface shape so to decide change of light propagating direction. We think that these analyses of light propagations in PCs in terms of equifrequency surface are of fundamental and practical importance in the miniaturization of optical devices such as wavelength multiplexers/demultiplexers.
