A CPU cooler includes a heat-conducting unit and a heat-dissipating unit. The heat-conducting unit consists of a heat-conducting block, a heat-conducting piping, an inflow pipe and an outflow pipe. The heat-conducting block is closely positioned on the CPU and the heat-conducting piping is disposed in the heat-conducting block for making liquid to flow therethrough. The heat-conducting piping has its inflow port connected with the inflow pipe and its outflow port connected with the outflow pipe, and the inflow port of the heat-conducting piping is positioned lower than the outflow port. The heat-dissipating unit has one end communicating with the outflow pipe and the other end communicating with the inflow pipe for dissipating the heat and lowering the temperature of the liquid, able to incessantly repeat the cycle of heat dissipation and temperature reduction.