Two-phase　flow　patterns　in　two　horizontal　microchannels　with　uniform　reentrant　cavities　in　sidewall　were　experimentally　studied,　using　deionized　water　and　air　as　working　fluids.　The　hydraulic　diameter　of　microchannels　in　the　constant　rectangular　cross-section　is　133.3　mu　m.　Two　structures　(fan-shaped　and　triangular)　of　reentrant　cavity　are　arranged　to　be　mounted　in　sidewall　with　0.25　mm　spacing　to　develop　two　new　microchannels　with　variable　cross-sections.　The　liquid　and　gas　superficial　velocities　in　the　microchannels　cover　the　0.1-0.5　m/s　and　0.1-10　m/s　ranges,　respectively.　High-performance　CCD　digital　imaging　camera　with　a　Nikon　microscope　is　used　to　determine　the　flow　patterns　in　the　two　new　proposed　microchannels.　Two　distinct　flow　patterns　were　observed:　intermittent　flow　and　separated　flow.　Evolution　characteristic　for　different　flow　patterns　along　the　flow　direction　are　presented.　Flow　pattern　maps　using　gas　and　liquid　superficial　velocities　as　coordinates　and　the　correlations　to　describe　flow　pattern　transition　for　the　newly　proposed　microchannel　test　sections　are　obtained,　respectively.　(C)　2013　Elsevier　Inc.　All　rights　reserved.