In this study, the hydrodynamic behavior and flow patterns of vertical upward gas-liquid flow have been studied. A bundle of square glass capillaries was used to observe and evaluate the hydrodynamic behavior of flow. The flow pattern was found by using the flow visualization technique. The effects of gas flow rate, size of capillaries and phenol concentration upon the flow regime were studied. The trends of the transitions were more evident with respect to the change of gas flow rate than that of liquid flow rate. The range of gas Reynolds numbers supporting a well-developed slug flow was relatively narrow and almost independent on the liquid flow rate. For three different concentrations, almost the same flow patterns were obtained. Finally, the effects of capillary site, surface tension, and flow rates were combinded by using dimensionless groups such as Froude number and Eotvos number.