Abstract: The expression of free energy is expanded in a power series, in which there aren ’t any terms of order higher than third in the temperature increments θ and second in the strains γii in this paper. The regular patterns of the material coefficients changing with temperature increments can be derived from this expression. These regulations accord with the experimental graph in references but the constants in the expression of free energy must be determined by experimental data.It is pointed out that the variable modulus of elasticity E and shearing modulus of elasticity G are independent of each other, but the rest of the coefficients are related to them.