Energy consumption in data centers has seen a drastic increase in recent years and approximately 40% of it is spent on cooling facilities. In data centers, server racks are cooled down indirectly by chilled air with an air conditioner. This technique is inefficient because IT equipment is not cool enough whereas the server room is overcooled. Under these circumstances, the authors have proposed liquid cooling systems in which CPUs are cooled down directly and exhaust heat is not radiated into the server room. Three cooling technique for CPUs are developed simultaneously. Two of them involve a single-phase heat exchanger and a two-phase heat exchanger as a cooling jacket. The other is flat heat pipes in order that the heat generated by CPU is cooled after being transported to the outside of the chassis. Condensation section of the heat pipe is cooled down by liquid with liquid manifold, so that there is no air cooling part in our new liquid cooling systems. Performance evaluation tests of the cooling devices have been conducted, which are single-phase heat exchanger, two-phase heat exchanger and two-types of flat heat pipes. Meanwhile, nanofluid technology for heat transfer enhancement and development of plug-in connectors for standardization of the systems are pursued. In addition, Verification tests have been carried out with using commercial servers. As a result, a 44% to 53% reduction in energy consumption of cooling facilities with single-phase cooling system was realized compared with conventional air cooling system.