Abstract: The paper describes a study on the vacuum brazing of aluminum alloy in nonbrazing sheet form.The effect of additional elements on wettability of Al-Si base filler alloy was determined.The satisfactory result was obtained by addition of 1~1.5% Mg.A small amount of bismuth can further improve the wetlability of filler alloy and reduce the requirement of vacuum degree for brazing.When brazing was carried out by placing the workpiece in a box inside a vacuum furnace, the brazing quality may be improved distinctly.In order to make filler alloy gap-filling flow, it is necessary to add a little magnesium in brazing zone.The role of magnesium during brazing process was studied.The magnesium vapor penetrated the fissure of oxide film and allcyed with aluminum.The filler alloy, after being melted, spreaded over this alloying surface and raised the oxide film, thus wetting occured.The superior brazing joint can be obtained by the vacuum brazing.