Recently,　microcavities　have　become　a　central　feature　of　diverse　microfluidic　devices　for　many　biological　applications.　Thus,　the　flow　and　transport　phenomena　in　microcavities　characterized　by　microvortices　have　received　increasing　research　attention.　It　is　important　to　understand　thoroughly　the　geometry　factors　on　the　flow　behaviors　in　microcavities.　In　an　effort　to　provide　a　design　guideline　for　optimizing　the　microcavity　configuration　and　better　utilizing　microvortices　for　different　applications,　we　investigated　quantitatively　the　liquid　flow　characteristics　in　different　square　microcavities　located　on　one　side　of　a　main　straight　microchannel　by　using　both　microparticle　image　velocimetry　(micro-PIV)　and　numerical　simulation.　The　influences　of　the　inlet　Reynolds　numbers　(with　relatively　wider　values　R-e　=　1-400)　and　the　hydraulic　diameter　of　the　main　microchannel　(D-H　=　100,　133　mu　m)　on　the　evolution　of　microvortices　in　different　square　microcavities　(100,　200,　400　and　800　mu　m)　were　studied.　The　evolution　and　characteristic　of　the　microvortices　were　investigated　in　detail.　Moreover,　the　critical　Reynolds　numbers　for　the　emergence　of　microvortices　and　the　transformation　of　flow　patterns　in　different　microcavities　were　determined.　The　results　will　provide　a　useful　guideline　for　the　design　of　microcavity-featured　microfluidic　devices　and　their　applications.