Abstract: In order to investigate the influence of ultrasonic on flow behavior of filler metal when the filler metal propagates on the solid/liquid surface in brazing process,the gap-filling behavior of the molten filler metal during ultrasonic-assisted brazing of magnesium alloy was in-situ observed by high-speed video camera. Besides,the gap-filling behavior in unparalleled gaps and brazed joint properties were investigated. When the filler metal fills the gap in the direction parallel to the ultrasonic energy propagation,the dynamic curve of filling-gap distance appears linear relation with the ultrasonic time. And it shows that at the same ultrasonic time,good pre-clearance results in low filling velocity. Along the filler flow direction,the thickness of brazed joint decreases gradually. The filling-gap distance decreases as ultrasonic time increases when filler metal is placed at large gap side. The compactness of the joint is general. The filling-gap distance increases firstly and then decreases with the ultrasonic time increasing when the filler metal is placed at small gap side. In this case,the defects appear in the whole joint. The analysis indicates that the flow behavior of filler metal in unparalleled clearance is influenced by the combined effect of ultrasonic induced gap-filling action and capillary action.