An　experimental　and　theoretical　research　has　been　carried　out　for　vertically　upward　two-phase　flow　of　air-water　across　staggered　rod　bundles　having　inclination　angles　of　0　degrees,　45　degrees　and　90　degrees.　New　correlations　for　single-phase　pressure　drops　in　rod　bundles　for　various　inclination　angles　were　developed.　The　single-phase　frictional　resistance　in　the　vertical　flow　was　found　to　have　a　minimal　effect　on　the　total　flow　resistance　in　the　inclined　flow.　Based　on　the　single-phase　pressure　drop　data,　a　new　superposition　model　was　proposed　and　verified　to　be　successful　in　predicting　both　the　present　data　and　the　experimental　data　in　previous　experiments　of　other　researchers.　The　two-phase　friction　multiplier　in　the　inclined　rod　bundle,　which　decreased　with　increasing　mass　velocity　at　a　given　value　of　Martinelli　parameter,　could　be　fit　well　in　terms　of　Martinelli　parameter　and　the　defined　dimensionless　mass　velocity.　In　addition,　the　two-phase　friction　multiplier　first　increased　and　then　decreased　with　increasing　Martinelli　parameter　at　a　given　value　of　mass　velocity　for　G　＜　200　kg　m(-2)　s(-1).　The　new　correlations　developed　for　two-phase　friction　multipliers　were　successful　in　predicting　the　two-phase　pressure　drop　data　in　the　inclined　rod　bundle.　(C)　2016　Elsevier　Inc.　All　rights　reserved.