Guided bands of two-dimensional photonic crystal waveguides are theoretically studied by using the plane wave expansion method. The photonic crystal waveguide is formed by removing rows of air holes in a triangular lattice air hole structure. Guided modes originate from the photonic band gap effect. Near Brillouin zone edges and band starting points, guided modes show very large group velocity dispersions. In some frequency regions, group velocity dispersions are over $10^6$ times larger than that of conventional optical fibers. Photonic crystal waveguides are very attractive candidates for dispersion controlling devices.