The relation of normalized core radius (p/Λ) with hole-to-pitch ratio (d/Λ) and normalized frequency (Λ/λ) is obtained using full-vectorial effective index method. The illogical fixed valve of p/Λ is modified, and an improved full-vectorial effective index method is proposed. The modal properties computed using this method, such as the effective index and total dispersion, closely agree with those of the multi-pole method as well the experimental values, and the error of effective index was reduced by two to three orders. The dispersion of photonic crystal fiber is analyzed in detail, and the relations of dispersion with wavelength, hole pitch and d/Λ is obtained.