  The effective potential of electroweak theory with two massless Higgs
doublets at finite temperature is studied. We investigate phase structure and
critical temperature in this model by numerical analysis without
high-temperature expansion. The phase transition is found to be of first order.
The critical temperature is shown to be relatively low for typical scalar
masses. The free energy of the critical bubble is calculated with some
approximations and we find that the bubble nucleation can occur at a
temperature a little below the critical temperature. We also discuss the
possibility of the electroweak baryogenesis.
