Experiments of air-water two phase heat transfer and frictional pressure drop without and with polyacrylamide (PAM) additive in a horizontal smooth circular tube were conducted. A PAM water solution with a concentration of 300 ppm was used in the experiments. The test tube has an inside diameter of 40 mm and an outside diameter of 48 mm. Heat transfer tests were performed by cooling air-water flow inside the test tube through its wall using cooling water with a heat transfer length of 3 m and two phase pressure drops were measured over a length of 3.1 m. The liquid phase superficial velocity V-LS = 0.52, 1.02 and 1.46 m/s and the gas phase superficial velocity V-GS = 1.66-4.9 m/s. The average volume void faction beta = 0.532-0.904. The total mass flux G = 5199-15,234 kg/m(2) s and the gas phase Reynolds number Re-G = 3892-12,025. It shows that the air-water two phase pressure drops can be reduced from 31.9% to 54.7% while the two phase heat transfer coefficients can be reduced from 36.8% to 70.3% with the PAM additive. Furthermore, the effects of the PAM additive on the air-water two phase heat transfer and pressure drop characteristics have been analyzed. The observed flow patterns are all slug flows. New physical mechanisms of the two phase pressure drop and heat transfer reductions have been proposed. (C) 2012 Elsevier Ltd. All rights reserved.