Stable Al2O3-water and TiO2-water nanofluids are prepared, and the morphology of nanoparticles and the stability of nanofluids are analyzed. An experimental set for heat transfer characteristics of CPU cooled by nano fluids is established in this paper. The heat transfer and flow characteristics of nanofluids in the CPU heat sink are experimentally studied. The effects of nanoparticle mass fractions (Al2O3-water: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 1.5% and 2%; TiO2-water: 0.1%, 0.2%, 0.3%, 0.4%, 0.5% and 1%) and Reynolds numbers (Re: 146, 275, 406, 479, 541, 593, 707 and 812) on the heat transfer and flow characteristics are discussed, In addition, the comprehensive evaluation for thermo-hydraulic performance of nanofluids is investigated. It is found that the nanofluids with the highest nanoparticle mass fraction have not the best heat transfer performance, and Nusselt number firstly increases and then decreases with nanoparticle mass fraction, and the critical nanoparticle mass fractions for the best heat transfer performance of Al2O3-water and TiO2-water nanofluids are omega = 1.0% and omega = 0.4% respectively. It is also found that Al2O3-water and TiO2-water nanofluids can reduce the temperature of CPU by 23.2% and 14.9% at best compared with water respectively.