Transient　behaviors　of　start-up　flow　from　rest　in　long　microcavities　at　low　Reynolds　numbers　(Re)　were　experimentally　studied,　using　a　high-speed　microscopy　system　and　a　micro-particle　image　velocimetry　(micro-PIV).　The　effects　of　the　Re　(1-116.9)　and　microcavity　aspect　ratios　(e　=　1,　3,　5)　on　the　developing　phenomena　of　dye　flow　patterns　and　vortex　structures　were　studied.　The　separatrix　morphologies　were　visualized　and　the　formation　and　expansion　of　the　vortices　were　characterized　to　reveal　the　mechanism　of　separatrix　migration.　Interestingly,　with　time　increasing　the　microcavity　flow　(e　=　3)　at　Re　=　50　experiences　three　forms,　namely　attached,　transitional,　and　separated　flow.　The　start-up　time　for　the　microcavity　flow　to　develop　into　a　steady　state　was　studied　for　the　first　time.　Developed　vortices　structures　were　also　measured　and　compared　with　developing　vortices　to　deepen　the　fundamental　understanding　of　vortices　behaviors.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.