A nanometer heat-conducting water solution for use in a car cooling system is disclosed. The nanometer heat-conducting water solution is formed by mixing a AlO.sub.2 solution (1.1 vol %) having 3-10 nanometer scale materials with a TiO.sub.2 solution (1.1 vol %) having 3-10 nanometer scale materials, wherein the obtained solution is then mixed with a diluent (93 vol %), and dispersing agents (3.43 vol %) and an emulsifying agent (1.37 vol %) are thereafter added so as to disperse the AlO.sub.2 solution and the TiO.sub.2 solution in the diluent uniformly. When the obtained stable nanometer heat-conducting water solution is added to a water tank of the car, the TiO.sub.2 cleans limescale and the emulsifying agent adheres to wall surfaces of water jackets to allow AlO.sub.2 to release energy continuously. Moreover, the nanometer scale materials speed up the micro-explosion of the cooling water so as to optimum the cooling effect and increase the heat-dispersing efficacy significantly.