The complex refractive index distribution of photonic crystal fibers makes a numerical algorithm necessary. An effective index method based on weak-guidance approximation is often used to study the modal properties of such fibers including their dispersion properties. In this paper, a fully vectorial effective index method is formulated for photonic crystal fibers based on the solution of the vectorial characteristic equation and is used to compute the dispersion parameters of photonic crystal fibers. In analogy with step-index fibers, the effective index of the photonic crystal fiber cladding can be calculated simply by substituting the appropriate Bessel functions of the electromagnetic field into the vectorial characteristic equation for step-index fibers. The result obtained by the vectorial effective index method is in better agreement with those by the finite element methods and measured data.