Abstract: The influences of surfactant on the pressure drop in horizontal two-phase flow was investigated experimentally. Air/water and air/100 mg•kg^-1 sodium dodecyl sulphate (with 95% purity grade) aqueous solution were used as working fluids. The results presented a demonstration that the drag reductions depended on flow regimes. Drag reductions up to 83.3% for stratified flow, 83.2% for plug and slug flows and 31.2% for annular flow were observed in the experiment. The drag reduction was caused either by adding the surfactant itself or by the shift of flow regime due to the addition of surfactant.