An　experimental　study　was　conducted　on　the　air-water　two-phase　flow　patterns　and　pressure　drop　in　the　bed　of　rectangular　cross　section　containing　spheres　of　regular　distribution　Three　kinds　of　glass　spheres　with　different　diameters　(3mm,　6mm　and　8mm)　were　used　for　the　establishment　of　the　test　section.　By　means　of　visual　observations　of　the　two-phase　flow　through　the　test　section,　it　was　discovered　that　five　different　flow　patterns　occur　in　the　experimental　parameter　ranges,　namely　bubbly　flow,　bubbly-slug　flow,　slug　flow,　slug-annular　flow　and　annular　flow.　A　correlation　for　bubble　and　slug　diameter　in　packed　beds　was　proposed,　which　is　an　extension　of　the　Tung/Dhir　model,　Jamialahmadi＇s　model　and　Schmidt＇s　model.　Three　correlations　were　proposed　to　calculate　the　void　friction　of　flow　regime　transition　in　bubble　flow,　slug　flow　and　annular　flow　based　on　the　bubble　model　in　the　pore　region.　The　experimental　result　shows　that　the　modified　Tung　and　Dhir＇s　model　of　flow　pattern　transition　is　a　better　agreement　with　the　experimental　data　compared　with　Tung　and　Dhir＇s　model