A new kind of nanofluid was prepared with water and functionalized nanoparticles. The nanofluid can keep a long-term stability and the functionalized nanoparticles will not aggregate even at a high mass concentration. The functionalized nanoparticles have a good solubility in the base fluid. The functionalization was achieved by grafting silanes directly to the surface of silica nanoparticles. The synthesis process was introduced. The thermophysical properties of the new nanofluid were measured. The thermal conductivity of the functionalized nanofluid increases slightly

while its viscosity increases greatly in comparison with the base fluid.