Critical heat flux (CHP) and flow characteristics of subcooled flow boiling with water in narrow tubes were investigated experimentally under uniform heat flux conditions. There were two experimental conditions: high heat flux regions, and low heat flux regions, based on the CHP characteristics. The CHP at high heat flux regions was higher than the predictions made by existing empirical correlations. They occurred at small inside diameter, and short length tubes, as well as at high water mass velocity. The effect of tube inside diameter on the incipient boiling and the net vapor generation point, as well as on the two phase friction multiplier, was made clear from pressure drop characteristics. The decrease of friction pressure drop, and the increase of critical heat flux, with the decrease of tube inside diameter, are attributed to the decrease of void fraction in narrow tubes.