Effective Potential of $\lambda\phi^{4}_{1+3}$ at Zero and Finite Temperature

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.