An experiment has been carried out to study the phase split of water-nitrogen two-phase flow through a horizontal T-junction with a square cross section (500 mu m x 500 mu m), focusing on the effect of flow pattern. By comparing the results of slug, slug-annular and annular flows, it is shown that phase split characteristic of micro-T-junction highly depends on inlet flow pattern. When the inlet flow is a slug flow, it takes on gas rich in side branch. But when the inlet flow shifts to an annular flow, the side branch is rich in liquid. For an slug-annular flow, the curves show transitional characteristics. The effect of superficial velocities on the phase split of each flow pattern is considered as following: the liquid taken off decreases with an increase of liquid superficial velocity and increases with an increase of gas superficial velocity. The results have been compared to that of a 500 mu m T-junction with a circular cross section. It is found that the square T-junction shows more liquid taken off at slug flow and slug-annular flow. While, at annular flow, the liquid taken off increases slower at high gas taken off in square T-junction due to cross section effect. (C) 2011 Elsevier Ltd. All rights reserved.