In this paper, the design and development of a device for thermal diffusivity measurement of fluids exploiting the photoacoustic effect is presented. Compared to similar devices working only at room temperature, the version of the apparatus we present allows precise measurements of thermal diffusivity of fluids from room temperature to 70 °C. The reliability of the measurements is shown by the estimation of thermal diffusivity of well-known test liquids. The possibility to detect slight thermal diffusivity variations (around 1%) is verified by measurements on alumina based water nanofluids showing remarkable advantages in respect to alternative techniques.