  The optimized linear $\delta$-expansion is applied to the $\lambda \phi^4$
theory at high temperature. Using the imaginary time formalism the thermal mass
is evaluated perturbatively up to order $\delta^2$. A variational procedure
associated with the method generates nonperturbative results which are used to
obtain the critical temperature for the phase transition. Our results are
compared with the ones given by propagator dressing methods.
