Abstract: Two-phase frictional pressure drop of air-water mixture was studied in a narrow rectangular duct with cross-section 40 mm by 1.6 mm. The experimental results show that the conventional correlations fail to predict the two-phase frictional pressure drop in narrow rectangular duct; Lee-Lee correlation which is based on the rectangular duct gives a relatively good agreement with our experiment, but has lower values than the experimental data. Different modified Chisholm correlations were proposed in two regions which were distinguished by the ratio of liquid Reyonlds number and gas Reyonlds number. The modified correlation has a good agreement with the experimental data.