In　this　study,　the　particles　in　plume　were　in-situ　measured　based　on　the　self-mixing　effect　of　the　semiconductor　laser　for　revealing　the　plume　formation　during　fiber　laser　keyhole　welding.　The　plume　consisted　of　two　parts:　the　oscillating　part　at　the　bottom,　and　the　slender　part　shaped　like　focused　laser　beam　and　with　a　decreasing　ascending　velocity.　The　diameters　of　particles　in　plume　mainly　distributed　in　the　range　of　0.3　mu　m　similar　to　0.65　mu　m　(accounting　for　about　70　%).　Along　the　laser　beam　direction,　both　the　diameter　and　density　of　particles　in　plume　decreased　with　the　increase　of　the　height.　In　the　same　horizontal　measurement　plane,　the　particle　diameter　distributed　uniformly,　and　particle　density　decreased　sharply　when　the　monitoring　plane　further　from　the　laser　beam　center.　There　are　a　great　quantity　of　particles　in　the　oscillating　plume　caused　by　eruption　of　the　laser-induced　vapor　on　the　front　keyhole　wall,　and　that　formation　of　the　slender　plume　is　related　to　the　fact　that　when　the　oscillating　plume　is　erupting　along　the　laser　beam　direction,　particles　in　it　can　enter　the　laser　beam　to　move　in　the　laser　beam　direction　and　to　give　out　light　after　being　heated　by　the　laser　beam.