This study investigates the effect of injecting nanofluids containing nano-SiO2 as drag reducing agents (DRA) at different concentrations on the pressure drop of air-water flow through horizontal pipe. The test fluid used in this study was air-water with nano-SiO2 particles at 0.1%-1% mass concentration. The test sections of the experimental set-up were five pipes of the same length of 9 m with ID from 0.0127m-0.03175m (0.5 to 1.25 in). Airwater flow was run in slug flow regime under different volumetric flow rates. The results of drag reduction (eta%) indicated that the addition of DRA could be efficient up to some dosage. Drag reduction performed much better for smaller pipe diameters than it did for larger ones. For various nanosilica concentrations, the maximum drag reduction was about 66.8% for 0.75% mass concentration of nanosilica. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.