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High-temperature annealing behaviors of long-period fiber grating (LPFG) induced by high-frequency CO2 laser pulse in an endlessly single-mode photonic crystal fiber (ESM-PCF) are investigated in this paper. We find that the change difference of effective refractive index between the air holes cladding and the solid core of the ESM-PCF both in grating fabricating and in annealing processes with different mechanisms would cause resonant wavelength red shift. Through annealing the gratings under high temperature at 600°C-650°C, its stability, linearity, and repeatability to temperature and tensile strain responses under high temperature condition can be greatly enhanced. Our study shows that the changes in fiber glass structure would have significant influence on the optical properties of ESM-PCF LPFG and thus will help understanding of the forming mechanisms and physical characteristics of ESM-PCF LPFG. In addition, ESM-PCF LPFGs can be used as temperature or strain sensors after annealing at high temperature of 600°C-650°C.