Spatial　phase　distributions　for　various　flow　patterns　and　two　phase　flow　frictional　pressure　drops　of　air-water　flow　in　a　horizontal　helically　coiled　rectangular　tube　were　experimentally　investigated　at　the　liquid　and　gas　superficial　velocities　of　0.11-2　m/s　and　0.13-16　m/s.　Local　void　factions　in　the　helically　coiled　tube　were　measured　with　electric　conductivity　probes　and　the　corresponding　flow　regimes　were　recorded　with　a　high-speed　video　camera　simultaneously.　The　local　void　fractions　and　spatial　phase　distributions　for　various　flow　regimes　were　analyzed　according　to　the　physical　mechanisms.　The　spatial　phase　distributions　of　gas　liquid　two-phase　flow　in　the　helically　coiled　tube　are　mainly　affected　by　the　gravitational　and　centrifugal　forces.　The　measured　single-phase　flow　and　two-phase　flow　pressure　drops　have　been　analyzed　for　various　flow　regimes.　A　correlation　was　proposed　to　predict　the　friction　pressure　drop　of　single-phase　flow　in　the　helically　coiled　tube.　The　existing　gas-liquid　two-phase　pressure　drop　frictional　correlations　were　compared　to　the　experimental　frictional　pressure　drop　data.　The　Awwad　et　al.　correlation　[Int.　J.　Multiphase　Flow,　21　(1995),　607-619]　predicts　91　%　of　the　experimental　data　within　+/-　30　%.