This paper presents the effect of surfactant additives on frictional pressure gradient and gas-liquid interfacial structure for the air-water two-phase flow in a 13.5mm diameter horizontal pipe. The surfactant (CTAC) used in the experiments was n-hexadecyltrimethylammonium chloride mixed with a counter-ion sodium salicylate. In the slug flow, the addition of CTAC produces the pressure drop reduction up to 40% at a liquid Reynolds number of 6000. At high Reynolds number above 20000, the effectiveness of CTAC addition is not observed. In the annular and stratified flow, the liquid holdup is affected by the addition of CTAC, which increase in the frictional pressure gradient.