The dispersions of the fundamental cladding mode in photonic crystal fibers(PCFs)are computed with a vectorial effective index approach in this paper. By comparing with the results obtained from biorthonormal-basis method or scalar effective method,it is found that the results of this paper are in accordance with that obtained from the biorthonormalbasis method, and that the scalar effective method is a good approximation at a lower air-filling fraction or higher normalized wave-number A/λ. For high accuracy at larger air-filling fraction or lower normalized wave-number A/λ,it is necessary to consider the vectorial method. The relation between effective index of cladding and microstructure of PCFs is discussed. The computation and analysis in this paper can provide a theoretic reference for designing of PCFs.