Abstract: A visual experimental system is designed to study the flow patterns in a rectangular mini-channel. The forces on gas-liquid two phases in critical state are analyzed. A new flow regime transition criterion based on mechanical model is proposed, which is compared with the experimental data. The results show that the prediction accuracy of bubbly flow-confined bubble flow transition criterion is 93.94% and the prediction accuracy of confined bubble flow-annular flow transition criterion is 94.07%. The prediction result is in agreement with the experimental data.