A liquid cooler includes a liquid cooling head, a liquid cooling radiator and liquid delivery pipes connected between the liquid cooling head and the liquid cooling radiator. The liquid cooling radiator has a liquid pump box, a heat dissipation pipe set and a liquid return box. The liquid pump box is divided into a hot liquid chamber and a cold liquid chamber by an air barrier space. The space of the hot liquid chamber is smaller than that of the cold liquid chamber, so that the hot liquid quickly enters the heat dissipation pipe set through the liquid pump box and overcomes the problems that the hot liquid in the liquid pump box is easy to accumulate heat and conduct heat to the cold liquid.