This　paper　presents　investigations　on　the　effect　of　a　surface　active　agent　on　the　liquid　hold-up　of　gas-liquid　two-phase　flow　in　a　horizontal　pipe　using　the　electrical　conductive　tomographic　technique.　Air/water　and　air/100　mg/kg　sodium　dodecyl　sulphate　(with　95%　purity　grade)　aqueous　solution　are　used　as　working　fluids.　The　results　presented　a　demonstration　that　the　surfactant　has　no　effect　on　the　average　liquid　hold-up　for　the　air-water　stratified　smooth　flow　regime.　With　increasing　the　liquid　velocity,　the　stratified　wavy　flow　regime　is　observed　in　the　air-water　flow　system.　At　the　moment,　the　addition　of　surfactant　can　suppress　or　eliminate　the　interfacial　waves　and　make　the　liquid　hold-up　increased.　For　air-water　slug　flow,　adding　surfactant　can　decrease　the　liquid　hold-up,　especially　at　the　lower　gas　and　liquid　velocities.