The present paper proposes new correlations for the two-phase pressure drop through horizontal rectangular channels with small gaps (heights) based on 305 data points. The gap between the upper and the lower plates of each channel ranges from 0.4 to 4 mm while the channel width being fixed to 20 mm. Water and air were used as the test fluids. The superficial velocity ranges of water and air were (0.03-2.39 and 0.05-18.7 m/s, respectively. The atmospheric pressure condition was maintained throughout the experiments. In the present study, the two-phase frictional multiplier was expressed using the Lockhart-Martinelli type correlation but with the modification on parameter C. Effects of the mass Hus and the gap size were considered. The correlations with the modified C successfully cover wide ranges of the Martinelli parameter (X = 0.303-79.4) and the all-liquid Reynolds number (RELo = 175-17700) based on the hydraulic diameter within the deviation of +/- 10%.