The nucleation mechanism of gas bubbles in melts was studied theoretically by three representative models, uamely the homogeneous nucleation in liquid metal, heterogeneous nucleation on the flat surface or in conic pits of refractory inclusions, under the assumption that the state of the system's Gibbs free energy change being maximal is the critical state of bubble nucleation. Introducing the contact angle and considering the effect of interfacial tension on bubble morphology, the following conclusions can be drawn: the critical nucleation radius of bubbles in the three models is the same, which will be reduced with increasing gas pressure above the liquid metal. The diameter of embryo bubbles, which is also the minimum pore size in lotus-type porous metal ingots, has a magnitude of micrometers under the common pressure of 0.1—1.0 MPa for the Gasar process. For heterogeneous bubble nucleation in conic pits of inclusions, there exists an optimal cone apex angle corresponding to the smallest volume of the embryo bubble, which is independent of gas pressure above liquid metals and increases with the increase of the contact angle. For alumina inclusion in Mg, Al, Fe, Ni and Cu melts, the optimal apex angles are 8.0°, 19.5°, 20.6°, 42.7° and 51.5°, respectively. Among these three bubble nucleation models, homogeneous nucleation is the most difficult, heterogeneous nucleation in conic pits of inclusions is the easiest, and heterogeneous nucleation on the flat surface of inclusions is intermediate.