Nanoferrofluids of CoxFe3−xO4 were prepared by the chemical co-precipitation method by varying the value of x (0.2, 0.6 and 1.0 M). The structural and surface morphological investigations were done by X-ray diffraction (XRD) and SEM techniques respectively. The particle size calculated from the data of XRD, (3 1 1) plane revealed that the particle size increases with higher cobalt content and are in the range of 5 – 16 nm. The ultrasonic velocity of the aqueous carrier fluid and Cobalt ferrofluids was measured by varying the temperature from 30 – 70 ◦C. The ultrasonic velocities of magnetic nanoferrofluids decrease with concentration in the absence of magnetic field. The higher value for the nanoferrofluid’s velocity compared to that of the carrier liquid in the absence of a magnetic field shows the influence of dispersed particles on the velocity of ultrasonic propagation.