The Effective Potential for Composite Operator in the Scalar Model at Finite Temperature

We discuss the ${\varphi }^4$ and ${\varphi }^6$ theory defined in a flat $D$-dimensional space-time. We assume that the system is in equilibrium with a thermal bath at temperature ${\beta }^{-1}$. To obtain non-perturbative result, the $1\mathrm{/}N$ expansion is used. The method of the composite operator (CJT) for summing a large set of Feynman graphs, is developed for the finite temperature system. The ressumed effective potential and the analysis of the D=3 and D=4 cases are given.