A long-period fiber grating (LPFG) couples light from the core to the cladding, which is highly sensitive to micro-bends. In this paper a novel pressure sensor based on LPFGs incorporated with a strain diaphragm structure is proposed and demonstrated. It was found that when the pressure changed from 0 to 0.13 MPa, the resonant peak wavelength was red-shifted linearly about 1.8 nm, and the peak amplitude decreased uni-directionally up-to about 13 dB. By the method of detecting the change of the transmission output power at a fixed wavelength inside the rejection band, the value of pressure could be measured. The resolution of this device could reach to 10-4 MPa, which is suitable for real applications.