Abstract: A theoretical analysis is made, by applying linear stability theory, orl the model of gas-liquid stratified flow. In the model the effect of liquid viscosity is considered, and the principle given for judging the interphase structure of gas-liquid stratified flow. This principle, applicable for the flow patterns of fluid with various viscosity, is compared with Tailtel Principle. Due to its adequate consideration on the effect of viscosity, this principle is more applicable for the calculation of oil/gas mixed flow, and more reliable in choosing model equations of mixed flow when used as a judging basis for transform from stratified flow to other flow patterns.