The present study deals with pressure effects on the hydrodynamic characteristics of gas-liquid two phases within a T-junction microchannel. The operating pressure is in the range of 0.1 similar to 5.0 MPa. Nitrogen and de-ionized water are selected as the test fluids. The gas Weber numbers vary from 1.37 x 10(-5) to 3.46 at atmospheric pressure and from 1.70 x 10(-3) to 70.32 at elevated pressure, respectively. The liquid Weber numbers are in the range of 3.1 x 10(-3)similar to 4.9. The operating pressure plays an important role in gas-liquid two phases flow. Seven typical flow patterns such as bubbly flow, slug flow, unstable slug flow, parallel flow, slug-annular flow, annular flow, and churn flow are observed. Based on the force analysis of the gas and liquid phase in microchannel, the formation mechanisms of flow patterns are discussed at great length, and the flow pattern maps are divided into five regions using We(Gs) and We(LS) coordinates. These results are beneficial for future investigation to understand gas-liquid two-phase mass transfer and reaction characteristics in microchannel at elevated pressure. (C) 2012 Elsevier Ltd. All rights reserved.