The propagation properties of photonic crystal fibers (PCF) are discussed with effective-index model. The effective-index model of photonic crystal fiber was introduced at first. By solving the scalar wave equation within cladding of PCF, the effective index of the cladding was calculated. Applying theory of standard fiber, we may calculate the propagation properties of PCF as a step-index fiber. With this model, the effective index of PCF cladding with different structure parameters was discussed. The refractive index difference of the core and cladding increases as the wavelength increases, and the endlessly single-mode behaviour was illuminated. The mode refractive index and the waveguide dispersion of the fundamental mode in PCF was calculated. It is shown that the relationship between waveguide dispersion and the air hole pitch is in accord with the scaling properties of Maxwell equations, the waveguide dispersion is also critically dependant on the size of the air hole. These analyses indicated that PCF is a promising component for enhanced control of dispersion in optical communication system.