Sized-based　sorting　and　trapping　of　particles　and　cells　from　a　mixture　utilizing　a　hydrodynamic　microvortex　has　been　a　flourishing　area　of　inertial　microfluidics　in　recent　years.　From　the　point　of　view　of　fluid　mechanics,　many　fundamental　issues　remain　unrevealed　in　this　research　area.　Here,　using　a　high-speed　microscopic　imaging　system,　we　experimentally　investigated　the　formation　and　evolution　of　isolated　particle　recirculating　orbits　induced　by　a　hydrodynamic　microvortex　within　a　square　microcavity　(400　mu　m　x　400　mu　m).　The　influence　of　the　inlet　Reynolds　number　(Re)　over　a　wide　range　(88-244)　on　the　evolution　of　recirculating　orbits　of　particles　with　different　diameters　(d　=　10　mu　m　and　20　mu　m)　at　relatively　very　low　concentration　was　systematically　investigated　to　further　previous　studies.　We　also　observed　an　intriguing　phenomenon　that　a　larger　single-particle　(d　=　35　mu　m)　always　occupied　the　outer　orbit,　while　a　smaller　single-particle　(d　=　20　mu　m)　occupied　the　inner　orbits　at　Re　=　155.　This　result　is　contrary　to　previous　reports　and　we　explored　the　reason　for　it.　Moreover,　we　quantitatively　characterized　the　dimensionless　particle　orbit　areas　(A)　and　critical　inlet　Reynolds　number　(Re-c),　which　determines　the　formation　of　particle　orbits.　The　results　provide　further　insights　into　the　fundamental　understanding　of　particle　behaviors　of　trapping　and　orbiting　and　a　useful　guideline　for　microvortex-based　microfluidics　applications.