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.