Abstract : The research reported describes experimental and theoretical investigations of high power density evaporative spray cooling. Preliminary experiments demonstrating heat fluxes greater than 1,000 W/sq cm were conducted. Extensive laser phase Doppler measurements for spray characteristics were also taken. These measurements provided valuable insight into the heat transfer process. An in-depth analysis was conducted to determine the mechanisms responsible for critical heat flux. Theoretical modeling was also conducted to determine the most desirable heat transfer conditions. After analysis of these results, an improve experimental apparatus was designed and fabricated. The new apparatus provided greater experimental control and improve accuracy. new tests were conducted in which the critical heat flux was increased, and the heat transfer efficiency was greatly improved. These results are compared to those of previous researchers, and indicated substantial improvement. Keywords: Evaporate cooling; Spray cooling; High heat flux; Critical heat flux.