The propagation properties of the long-period fiber grating (LPG) pair in standard single-mode fiber (SMF) are discussed including some factors affecting the interference transmission spectra,and the characteristics of the LPG pair in the photonic crystal fiber (PCF) at the U-band near 1665 nm are investigated in detail. A dual resonance interference spectrum is obtained after the structural parameters of the LPG pair in PCF are designed based on the relation between coupling strength and the beat length as well as the relation between beat length and wavelength. The spectra at both resonance wavelength are different due to the different coupling strength.