  We discuss the $\phi^6 $ theory defined in $D=2+1$-dimensional space-time and
assume that the system is in equilibrium with a thermal bath at temperature
$\beta^{-1}$. We use the $ 1/N $ expansion and the method of the composite
operator (CJT) for summing a large set of Feynman graphs.We demonstrate
explicitly the Coleman-Mermin-Wagner theorem at finite temperature.
