Surface oxidation, as one of fundamental chemical reactions in metals, greatly affects their properties. Herein, we develop a new quaternary GaInSnZn liquid metal with the melting temperature of 9.7 °C, which is the lowest among all reported Ga-based liquid metals. With high-resolution transmission electron microscopy, we directly observed the oxide layer formed on the surface of the liquid metal. The initially formed oxide layer is revealed to be amorphous and very sensitive to electron beam. Prolonged irradiation results in its structural change from amorphous to crystalline phases. The present finding refreshes the basic understanding of surface oxidization of liquid metals and opens up the possibility of tuning surface structures and morphologies by using electron beam irradiation.