Ultrasonic vibration was applied to the surface of AZ91D/CNF slurries in order to homogeneously disperse the reinforcement through the semi-solid matrix, and eliminate the clusters of nano-fibres originated during previous processes. The method utilized to produce the slurry was a modified compocasting process that generates semi-solid magnesium alloy using an inclined cooling plate and mechanical stirring. The CNF reinforcement was added in quantities varying from 1 to 3vol% and different vibration times and amplitudes were applied to the slurries under isothermal conditions. Macro and microstructure combined analysis showed the occurrence of grain refinement in the whole ingot for higher wave amplitudes at shorter times, while for lower amplitudes longer times up to 10min. were required. Vibration times over 10min were not significant to further improve the results and times up to 4min did not seem to provide any result whatsoever. The best results, however, were obtained for 16 and 18μm amplitudes. Discontinuous application of ultrasound seems to produce better results than continuous vibration. The dispersion of reinforcement seems to occur by the smash of CNF clusters due to the movement of the solid fraction present in the Mg alloy slurry, and by cavitation, responsible also for some microporosity present in the microstructures.