A full-vector plane-wave method(FVPWM) is introduced to calculate the photonic crystal in-plane photonic bandgap of a 2-D triangular structure.Using the numerical simulation,we can obtain the in-plane bandgap structure of hollow-core photonic crystal fiber.In our experiments,we measure the transmission spectra of a hollow-core photonic crystal fiber and silica rod,and by using the numerical calculation the relative transmission spectrum of hollow-core photonic crystal fiber can be gotten.Compared with the theory simulation,we find that the experimental result is accordant with the simulation,they have the same bandgap in the same normalized frequency.