To improve the heat transfer performance during nucleate boiling applied to the cooling of semiconductor chips and of power electronics with high heat generation density, the present authors proposed a promising method of using immiscible mixtures. The nucleate boiling of immiscible mixtures with optimized fluid combination, composition and distribution of phases can make simultaneously possible i) drastic increase of CHF, e.g. 3×10^6W/m^2 for aqueous mixture without using enhanced surfaces, ii) reduction of surface temperature, iii) reduction of surface temperature overshoot at the incipience of boiling, and iv) elevation of system pressure preventing the mixing of air. Because detailed heat transfer characteristics remain unknown, the paper summarizes the recent results from pool boiling and flow boiling experiments performed in our laboratory.