High temperature fast annealing behavior of CO₂ laser pulse induced long-period fiber grating (LPFG) in an endless single-mode photonic crystal fiber (ESM-PCF) was studied.The experiment shows that the turning temperature is about 600 ℃.When the annealing temperature is lower than 600 ℃,the annealed ESM-PCF LPFG′s temperature resporse and tensile strain response are both unstable when the ambient temperature is lower than the anneaing temperature.And both the linearity and the repeatability of the responses are poor.When the annealing temperature is between 600 ℃ and 700 ℃,the annealed ESM-PCF LPFG′s temperaure response and tensile strain response are both stable with good linearity and repeatability when the ambient temperature is lower then the annealing temperature.When the temperature goes higher,the grating will be gradually and partly erased.This can be explained by the releasing stress induced in the grating writing process and fused silica structure change when heated.