The small size of microbubbles, along with their stability, has made them popular in many applications. Most of the applications involve the flow of microbubbles through capillary environments. In this study, the theological behavior of microbubbles has been examined using their flow index in capillary tubes, in Re < 50. The flow indexes of microbubbles have been calculated from the experimental pressure drop measured along the capillary tube. Microbubbles presented pseudoplastic behavior. Flow index was only influenced by bubble void fraction and did not show any dependency on the geometrical conditions of the tube. From the measured pressure drops along the capillary tubes, the Fanning friction factor has been calculated. It has been found that, in low Re numbers, Poiseuille's equation underestimates friction factor. In this work, for the first time,the non-Newtonian Re was employed to calculate the Fanning friction factor for microbubbles suspensions in capillary tube. A very good agreement between experimental data and the linear equation of friction factor with non-Newtonian Re has been observed. (C) 2014 Elsevier Ltd. All rights reserved.