This paper describes an experimental investigation which has been carried out with distilled water with the range of heat flux and pressure covering 7,400-28,000kcal/$m^2/h$ and 0.42-1.0332kg/$cm^{2}abs$, respectively. In this experiment, Nickel coated mirror surface heater of 5 cm O.D. was used as a heating source. The conclusions summerized as follows;1. The useful correlation of the test data for the heat transfer coefficient is presented as a function of the pressure. $$a/a_{s}=c{ imes}p;0.18$$ where a is the heat transfer coefficient and $a_s$ is the heat transfer coefficient at atmospheric pressure and p is the pressure, C is constant. 2. The bubble diameter near the heating surface and rising velocity increased with the heat flux. 3. A decrease in pressure results in the decrease of the number of nucleation sites and the increase of the bubble volume. 4. Bubble rising velocity differences are incrased maximumly up to $200\%$ of that at atmopheric pressure.