The group-velocity-dispersion properties of photonic crystal fiber (PCF) were investigated in detail by equivalent-index model. The PCF discussed here is made of single material (SiO2), the total dispersion is determined by waveguide dispersion, so the total dispersion has been divided into waveguide dispersion and material dispersion. The dependence of waveguide dispersion on structure parameters is investigated. It shows that when the relative hole size f is fixed, the relationship between waveguide dispersion and the pitch Λ accords with the scaling properties of Maxwell equations; when the pitch Λ is fixed, the zero and the extremum positions of the waveguide dispersion are linear with f in prefer wavelength range. As examples, how to design the structure parameters for engineering the chromatic dispersion of PCF is demonstrated.