We have experimentally studied how the Prandtl number (Pr) of a nanofluid depends on the concentration, size, and material of nanoparticles. The nanofluids were prepared using distilled water and nanoparticles of silica, alumina, titania, and zirconia. The volume concentration of particles was varied from 1 to 8% and their diameters changed from 10 to 150 nm. It is established that Pr values of nanofluids increase with the concentration of nanoparticles. The Prandtl number also significantly depends on the size of nanoparticles and decreases with increasing particle diameter.