Plenty of new symmetries have been proposed in various neutrino models. Phase transitions happen once these symmetries are broken. I will introduce the mechanism of leptogenesis via the phase transition. Without going into details of any neutrino mass models, I will focus on the effective Weinberg operator. During the symmetric phase transiting to the broken phase, the coupling of the Weinberg operator is time-dependent, and a lepton-antilepton asymmetry is generated by the interference of the Weinberg operator at different times. This mechanism differs from most works of leptogenesis in the literature, where the matter-antimatter asymmetry is generated from the decay or production of new particles. Slides can be found here.