We present a new application of a hot wire sensor for simultaneous and independent measurement of thermal conductivity k and diffusivity a of (nano)fluids, based on a hot wire thermal probe with ac excitation and 3 omega lock-in detection. The theoretical modeling of imaginary part of the signal yields the k value while the phase yields the alpha value. Due to modulated heat. flow in cylindrical geometry with a radius comparable to the thermal diffusion length, the necessary sample quantity is kept very low, typically 25 mu l. In the case of relative measurements, the resolution is 0.1% in k and 0.3% in alpha. Measurements of water-based Aerosil 200V nanofluids indicate that ultrasound treatment is more efficient than high pressure dispersion method in enhancing their thermal parameters.