The oxidation behviours of liquid tin with and without trace Ge at 250 degrees C in atmosphere were studied by both observation and removing dross from the surface periodically. The surface composition, atom's valence state and the surface morphology were researched by both XPS and SEM. The results showed that the liquid tin has an excellent oxidation resistance in atmosphere when the Ge content was up to 0.01% (mass fraction). XPS analysis presents that trace Ge segregated highly on surface, and existed as an oxide with positive four valences. Based on the results, a mechanism why trace Ge can improve the oxidation resistance of liquid tin was suggested, that is the trace Ge rapidly segregated on the surface of liquid tin when it was melting, then it formed a layer of protective and density oxide film to resist future oxidation of liquid tin in atmosphere.