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Strain and temperature characteristics of a long-period grating (LPG) written in an endless-single-mode photonic crystal fiber (ESM-PCF) are investigated theoretically and experimentally. By use of a dispersion factor , a deeper understanding of the behavior of LPG in the ESM-PCF is achieved. The negative strain sensitivity of the LPG is explained by the negative value of the dispersion factor . Our analysis clearly reveals the significant effect of the waveguide dispersive characteristics of the cladding modes on the strain and temperature characteristics of the LPG in the ESM-PCF. By selecting an appropriate grating period, a simple, low-cost LPG sensor with approximately zero temperature sensitivity but large strain sensitivity is realized.