  The effective potential of $\lambda\phi^4_{1+3}$ model with both sign of
parameter $m^2$ is evaluated at T=0 by means of a simple but effective method
for regularization and renormalization. Then at $T\ne 0$, the effective
potential is evaluated in imaginary time Green Function approach, using the
Plana formula. A critical temperature for restoration of symmetry breaking in
the standard model of particle physics is estimated to be $T_c\simeq 510$ GeV.
