This study deals with critical heat flux that occurs in boiling heat transfer while transient heat input as it increased in a exponential function with time in a pool of water and well wetting liquid, ethanol. The boiling heat transfer characteristics and CHFs were measured using a 1.0 mm diameter horizontal cylinder for wide range of pressures and subcoolings, with the photographic observations on the vapor bubble behavior by using a high-speed video camera. It was confirmed that CHFs changed systematically depending on experimental conditions. The quasi-steady-state CHFs measured were divided into two mechanisms for lower and higher subcooling resulting from HI or HSN. The dependency of HSN in the high subcooling was found remarkably in wetting liquid. Typical trends of the transient CHFs were clarified to three groups corresponding to time constant, periods