In parallel evaporative micro-channels. system instability may occur in terms of cyclical fluctuations at a long period This is due to the co-existence of the liquid phase flow at high Mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow rate pumping, with a pressure regulating tank and a constant heating pre-heater, alternations between these two states of boiling and non-boiling could happen with a period of minutes. This cyclical system instability has been modeled. where the liquid phase flow occurs at conditions of high inlet subcooling and low surface heat flux that the boiling inception is hard to initiate The system instability criteria are established in terms of a system binary states parameter, S. and a non-dimensional surface heat flux. This model has been validated experimentally. (C) 2009 Elsevier Ltd. All rights reserved.