“Endlessly single-mode” operation is the most remarkable characteristic of photonic crystal fibers. Birks et al. explained it reasonably by effective index model,but considerable error was engender in quantitative analysis,and the properties of photonic crystal fibers with larger filling ratio can't be analyzed by this method because of its scalar approximation. Effective index model is applied for modeling photonic crystal fibers,but improvement is made:(1)effective core radius and the normalized frequency which determines the second modal cutoff are more exact;(2)full vector method is used. The characteristic of mode cutoff of photonic crystal fibers is studied. The normalized frequency is calculated for a range of fiber parameters and wavelengths. Relative cutoff wave number for some fiber parameters is obtined,and the relative cutoff wave number and its asymptotic bound are used to determine the boundary of three operation regimes of photonic crystal fibers with endlessly single-mode,single-mode and multi-mode operation. The result obtained is in good agreement with numerical methods,and calculation is more convenient than numerical methods.