The mode field distributions are studied by using plane wave method. The influence of periodic structure, point defect and line defect on mode field distribution are discussed in detail. The results reveal that the mode field is a plane wave along k-direction modulated by a periodic function, just as indicated by Bloch theorem; the mode field is changed with k even with one band; mode field mainly locates in the areas of high dielectric constant, and shows a high degree of symmetry; mode fields in neighboring bands are orthogonal except those degenerate bands; the mode field shapes are relation with the lattice structure, the square lattice show square shape; the mode field distributions are different for different defect radius. Mode field distribute in the defect and its surrounding area and shows significant period symmetry.