Experiments　of　flow　regime　visualization　and　identification　of　air-water　two-phase　flow　were　conducted　in　a　horizontal　helically　coiled　rectangular　channel.　The　test　superficial　liquid　and　gas　velocities　are　0.09-2　m/s　and　0.18-16　m/s,　respectively.　Flow　regimes　were　observed　with　a　high-speed　video　camera　and　the　corresponding　local　and　average　void　fractions　were　measured　with　an　electric　conductivity　probe　method　and　with　a　quick-close　valve　method,　respectively.　Four　main　flow　regimes　including　unsteady　pulsating　flow,　bubbly　flow,　intermittent　flow,　and　annular　flow　were　observed.　The　bubbly　flow　identification　criteria　depend　on　more　than　90%　bubbles　whose　chord　length　was　smaller　than　the　channel　equivalent　diameter.　The　annular　flow　identification　criteria　is　the　local　void　fraction　in　the　gas　core　larger　than　0.97.　Then　the　flow　regimes　and　their　transition　mechanisms　are　analyzed.　Furthermore,　new　transition　criteria　among　these　flow　regimes　have　been　proposed.　The　results　show　that　the　critical　transition　average　void　fraction　from　bubbly　to　intermittent　flow　is　0.23.　A　complete　air-water　flow　regime　map　has　been　developed　for　the　horizontal　helically　coiled　rectangular　channel　and　the　map　predicts　the　observed　flow　regimes　well.