In the simulation of photonic crystal fibers, the fixed value of the normalized equivalent core radius \left(p/\Lambda \right) in the fully vectorial effective index method is inaccurate. For what is believed to be the first time, the relation of p/\Lambda with normalized frequency \left(\Lambda /\lambda \right) and hole-to-pitch ratio \left(d/\Lambda \right) is obtained. The improved fully vectorial effective index method (IFVEIM) is proposed. The modal properties computed with the IFVEIM, such as the effective index and the total dispersion, are closely matched to those of the multipole method as well as to experimental values, and the errors of the effective index have been reduced by 2 to 3 orders of magnitude.