By using the plane-wave expansion method, the effect of point group symmetry on irreducible Brillouin zone in two-dimensional photonic crystals is studied. Additional cylinder in unit cell of single-cylinder two-dimensiional photonic crystal is designed as symmetrical structures with different operation numbers of symmetry, eigen-frequencies of the five lowest energy bands in the Brillouin zone are calculated. The results show that in the calculation of two-dimensional photonic crystal band structure, if the photonic crystal point group symmetry operation number is n, the irreducible Brillouin zone should be 1/n of the whole first Brillouin zone.