With　air-water　as　working　fluid,　the　gas-liquid　two-phase　flow　pattern　and　evolvement　characteristics　in　a　microchannel　with　periodic　expansion-constriction　cross　sections　were　investigated　experimentally　by　means　of　IDT　high-speed　camera　mounted　together　with　a　Nikon　microscope.　The　observed　flow　patterns　were　intermittent　flow　and　separated　flow.　For　intermittent　flow,　gas　scattered　in　liquid　phase　or　liquid　scattered　in　gas　phase,　and　the　two　forms　appeared　in　turn.　For　separated　flow,　gas　flowed　mainly　along　the　wall　side　of　gas　inlet,　and　liquid　flowed　mainly　along　the　liquid　side　of　water　inlet.　There　was　an　obvious　interface　between　two　phases　and　the　interface　fluctuated　along　the　flow　direction.　According　to　gas　phase　and　liquid　phase　superficial　velocities,　flow　pattern　maps　were　drawn　and　correlations　for　describing　flow　pattern　transition　in　the　newly　proposed　microchannels　were　obtained.　It　was　found　that　for　a　given　liquid　phase　superficial　velocity,　the　flow　pattern　transition　gas　phase　superficial　velocity　for　the　microchannel　with　triangular　reentrant　cavities　was　slightly　smaller　than　that　for　the　microchannel　with　fan-shaped　reentrant　cavities.