With　the　aim　of　deepening　the　fundamental　understanding　of　particle　flow　behaviors　in　inertial　microfluidics,　a　mechanism　of　collision-triggered　particle　trapping　in　a　confined　rectangular　microcavity　(400　x　400　mu　m(2))　using　microvortices　is　proposed,　and　an　intriguing　phenomenon　that　the　orbit　area　of　large　particles　is　larger　than　that　of　small　particles　(diameter　range　of　22-38　mu　m)　under　the　same　flow　conditions　(Reynolds　number　=　178)　is　observed,　which　is　in　contrast　to　that　indicated　in　previous　reports.　Moreover,　the　flow　field　structures　of　the　microvortices　are　studied　by　micro-particle　image　velocimetry　(micro-PIV),　and　the　rotating　behavior　of　a　single　particle　(diameter　=　40　mu　m)　during　orbiting　is　first　measured　experimentally.　The　results　are　expected　to　provide　useful　guidelines　for　the　applications　of　microcavity-based　microfluidics.　(C)　2017　The　Japan　Society　of　Applied　Physics