Abstract: In some investigations on variational principle for coupled thermoelastic problems, the free energy Φ(e_ij,θ),where the state variables are elastic strain e_ij and temperature increment θ, is expressed by This expression is employed only under the condition of |θ|<0(absolute temperature of reference) But the value of temperature increment is great, even greater than T₀ in thermal shock. And the material properties (λ,μ,ν,c, etc.) will not remain constant, they vary with θ. The expression of free energy for this condition is derived in this paper. Equation (0.1) is its special case.Euler's equations will be nonlinear while this expression of free energy has been introduced into variational theorem. In order to linearise, the time interval of thermal shock is divided into a number of time elements Δt_k, (Δt_k=t_k-t_k-1,k=1,2…,n), which are so small that the temperature increment θ_k within it is very small, too. Thus, the material properties may be defined by temperature field T_k=T(x₁,x₂,x₃,t_k-1) at instant t_k-1, and the free energy Φ_k expressed by eg. (0.1) may be employed in element Δt_k.Hence the variational theorem will be expressed partly and approximately.