With a fully vectorial effective index method, the total dispersion coefficient of photonic crystal fiber is calculated, and it is deeply analyzed that how the total dispersion coefficient is affected by the structural parameters. It is found that the dispersion coefficient has two-extreme characteristics with the changes of structural parameters. 1) When Λ is unchanged, with the reduction of d/Λ, the zero-dispersion wavelength (ZDW) moves to the long wavelength. After reaching the maximum ZDW (MZDW), instead, the ZDW moves to the short wavelength. For example, when Λ=2.3 μm, the MZDW is about 1728.9 nm, as d/Λ=0.24, and the smaller Λ has the longer MZDW than the larger. 2) When d/Λ is unchanged, with the reduction of Λ, the ZDW moves to the short wavelength. After reaching the minimum ZDW (M-ZDW), the ZDW moves to the long wavelength. For example, when d/Λ=0.9, the M-ZDW is about 564.29 nm, as Λ=0.6 μm, and the larger d/Λ has the shorter M-ZDW than the smaller. These discoveries could be used to optimize design of the special photonic crystal fiber.