Abstract: Two-phase frictional pressure drop gradients of air-water mixture were studied in two vertical narrow rectangular ducts (40 mm×1.6 mm and 40 mm×3 mm). The experimental results show that the conventional correlations fail to predict the two-phase flow pressure drop in narrow rectangular duct with the 1.6 mm gap, but can predict the two-phase pressure gradient satisfactorily in rectangular duct with 3 mm gap except Friedel model and Tran model. Different modified Chisholm correlations were proposed in two regions which were distinguished by the liquid Reyonlds number. The results show that the constant C has a linear relationship with the hydraulic diameter, and C decreases with the increase of the hydraulic diameter. The new correlations gave accurate predictions of the pressure drop.