A cooling jacket for electronics devices, composed of a main heated channel with grooves and auxiliary unheated channels for additional liquid supply, was developed. The structure prevents dryout phenomena, i.e. the heat transfer limitation in boiling, by the reduction of substantial heated length. The heating surface, with a length of 30mm and a width of 30mm, was installed horizontally facing upwards in the cooling jacket. Test liquid was selected as FC72 to keep electric insulation from semiconductors integrated in the server. The measured critical heat flux (CHF) was larger than 33.3×10^4W/m^2 (300W) for inlet volumetric flow rate V_ = 0.8l/min and 1.0l/min (mass velocity G_ = 372kg/m^2 and 466kg/m^2s, respectively) at inlet liquid temperature T_ = 35℃ and test pressure P=0.15MPa. Larger CHF values compared to those for the channel without auxiliary unheated channels were confirmed.