General relation of temperature characteristics for long-period fiber grating (LPG) is obtained from deduction by using mode coupling theory. Thermal-optics coefficient differences of guiding mode in fiber and coupled different cladding mode are determined through measurement of temperature characteristics for LPG with period of 400～600 μm. The relations of temperature sensitivity with grating periods and coupled cladding mode orders are summarized combining the formula of temperature characteristics for LPG. The method of improving temperature stability for LPG is presented on the basis of theory along with experiments.