Fluid　flows　in　microchannels　with　microcavities　at　low　Reynolds　number　are　increasingly　used　in　microfluidic　applications　such　as　trapping　and　sorting　of　cells　and　particles.　For　optimizing　the　microcavity　configuration　and　better　controlling　the　microenvironment　in　the　microcavities,　it　is　important　to　thoroughly　understand　the　flow　behaviors　in　the　microcavities.　Hence,　using　microparticle　image　velocimetry　(mu　PIV),　we　investigated　quantitatively　the　flow　characteristics　of　rectangular　microcavities　with　a　wide　range　of　aspect　ratio　(lambda　=　0.25-3)　and　Reynolds　numbers　(Re　=　0-100).　Depending　on　the　control　parameters　(Re　and　lambda),　a　flow　regime　map　in　microcavities　has　been　constructed,　including　three　different　flow　patterns:　attached　flow,　separated　flow,　and　transitional　flow.　The　critical　parameters　for　the　transform　of　flow　patterns　were　determined.　Only　a　single　central　microvortex　appears　in　the　microcavities,　and　the　evolution　and　characteristics　of　the　microvortex　were　investigated　in　detail.　The　results　revealing　the　flow　mechanism　of　different　flow　patterns　in　the　rectangular　microcavities　can　provide　useful　design　guidelines　of　microfluidic-based　devices,　as　well　as　a　map　to　help　microfluidic　users　in　their　design　of　application-driven　microcavities.