Void　fraction　is　an　important　parameter　in　designing　and　simulating　the　relevant　gas-liquid　two-phase　flow　equipment　and　systems.　Although　numerous　experimental　research　and　modelling　of　void　fraction　in　straight　circular　channels　have　been　conducted　over　the　past　decades,　the　experimental　data　and　prediction　methods　for　the　average　void　fraction　in　helically　coiled　channels　are　limited　and　needed.　Especially,　there　is　no　such　information　in　helically　coiled　channels　with　rectangular　cross　section.　Therefore,　it　i;　essential　to　advance　the　relevant　knowledge　through　experiments　and　to　develop　the　corresponding　prediction　methods　in　helically　coiled　rectangular　channels.　This　paper　presents　experimental　results　of　the　average　void　fraction　and　new　models　for　the　void　fraction　in　a　horizontal　helically　coiled　rectangular　channel.　First,　experiments　were　conducted　with　air-water　two-phase　flow　in　the　horizontal　helically　coiled　rectangular　channel　at　a　wide　range　of　test　conditions:　the　liquid　superficial　velocity　ranges　from　0.11　to　2　m/s　and　the　gas　superficial　velocity　ranges　from　0.18　to　16　m/s.　The　average　void　fractions　were　measured　with　a　quick-closing　valve　(QCV)　method.　The　measured　void　fraction　ranges　from　0.012　to　0.927　which　cover　four　flow　regimes　including　unsteady　pulsating,　bubbly,　intermittent　and　annular　flow　observed　with　a　high　speed　camera.　Second,　comparisons　of　the　entire　measured　average　void　fraction　data　to　32　void　fraction　models　and　correlations　were　made.　It　shows　a　low　accuracy　of　these　models　and　correlations　in　predicting　the　experimental　data　for　the　void　fraction　smaller　than　0.5　while　the　drift　flux　model　of　Dix　(Woldesemayat　and　Ghajar,　2007)　predicts　98.3%　of　the　entire　experimental　data　within　+/-　10%　for　the　void　fraction　larger　than　0.5.　Therefore,　the　Dix　model　is　recommended　for　the　void　fraction　larger　than　0.5.　Furthermore,　the　observed　flow　regimes　in　the　coiled　channels　were　compared　to　two　mechanistic　flow　regime　maps　developed　for　horizontal　straight　circular　tubes.　The　flow　regime　maps　do　not　capture　all　flow　regimes　in　the　present　study.　Finally,　the　effects　of　the　limiting　affecting　parameters　on　the　void　fraction　models　are　analyzed　according　to　the　physical　phenomena　and　mechanisms.　Incorporating　the　main　affecting　parameters,　new　void　fraction　models　have　been　proposed　for　the　void　fractions　in　the　ranges　of　0　＜　alpha　＜=　0.2　and　0.2　＜　alpha　＜=　0.5　respectively　according　to　the　slip　flow　model.　Both　models　predict　the　experimental　data　reasonably　well.　Overall,　the　new　proposed　models　and　the　recommended　model　predict　92.8%　of　the　entire　void　fraction　data　within　+/-　30%.