The properties of long period gratings in photonic crystal fibers (PCFLPG) are systematically analyzed by the improved full-vector effective index model and the coupled mode theory. Dual-resonance peak occurs in PCFLPG. The dependence of the coupling strength and the resonance wavelength on the hole pitch, the hole-to-pitch ratio, the grating period, the length of grating, the refractive index modulation depth and the chirp coefficient is studied. The results provide the theoretical basis for the applications of PCFLPG in the optical fiber communication, the optical fiber sensing and so on.