Thermal diffusivity of a ferroelectric based electro-rheological fluid is studied. The rheological fluid is prepared by dispersing powders of Bi 4 Ti 3 O 12 in silicon oil. The thermal diffusivity of the sample, under different intensities of applied electric field, was measured by means of an open cell photo-acoustic technique. We have found that the main contribution to the measured photo-acoustic signal, comes from the thermo-elastic bending effect. It was clearly observed that thermal diffusivity increases with the intensity of the applied electric field, this fact is explained in terms of the formation of clusters of Bi 4 Ti 3 O 12 particles.