Heat transfer mechanism during flow boiling of fluids in small channels differs significantly depending on whether two-phase flow is in slug or annular regime. Understanding of the transition conditions between homogeneous slug flow and annular two-phase flow is an important topic for mini-and microchannel heat exchangers performance optimization. The current study focuses on the analysis of thermodynamic equilibrium conditions of two neighboring two-phase flow regimes. In both flow patterns the total energy is equal at specific mass flux and vapor quality and those values can be used to mark the transition conditions.