Abstract: Based on morphology, it is presented that morphotype conversion of foams in metallurgical melts would occur during foam evolution, which has been proved by experimental results. By means of morphotype conversion, a new interpretation on foams decay process was provided for promotion of kinetics on foaming. Characteristic numbers Ga and factor Sa were employed to be as criterion to qualitatively determine the decay rate before and after the morphotype conversion, respectively. The reasons also were given on contrast in foams stability from different liquid and gas source.