The effective index of fundamental mode and effective cladding index for index-guiding photonic crystal fibers with triangular lattice are calculated by using finite-difference beam propagation method and plane wave expansion method, respectively. Then the equivalent core radius for photonic crystal fibers is obtained through putting them into the mode dispersion equation for step index fibers. Variational regularity of the normalized equivalent radius(r norm=r/Λ) with normalized frequency(Λ/λ) is found out for different photonic crystal fiber parameters. Finally, the approximate formula for r norm as a function of d/Λ and Λ/λ, suitable to 2≤Λ/λ≤10 and 0.2≤d/Λ≤0.7, is given out.