Researches concerning micro actuators utilizing vapor-liquid interfacial phenomena are extensively investigated to develop thermal devices applied to micro machines. On the other hand, the application of two-phase flow is useful for the removal of waste heat from the semiconductor chips with highly increased heat generation density to be integrated in notebook PCs. In the present paper, the latest Japanese research on boiling and two-phase flow in mini channels is reviewed covering those for the fundamental phenomena and practical applications. Boiling in a narrow channel between parallel plates is an ideal system for the development of the high-performance heat exchangers with extremely small sizes. The promising approaches to increasing the critical heat flux are introduced including those by the present author to compensate the disadvantage inherent in this system.