Flowability of micrometer-sized particles treated with nanometer-sized particles has been studied by a vibrating capillary method. Two different types of treatments, i.e. dry particle coating based on the mechanofusion and simple mixing through shaking manually, were adopted to prepare composite particles and powder mixtures, respectively. The micrometer-sized core particle was polymethylmethacrylate (PMMA), and the coating nano-particles were TiO2, Al2O3, or SiO2. Surface morphology of the treated particles was analyzed by digital processing of SEM photographs, and flow behavior of powders in the vibrating capillary tube was studied with particular attention to the effects of the kind of nano-particles, its concentration, and the treatment method. Two factors were introduced to evaluate the flowability; one is the critical amplitude of vibration making the powder flow, and the other is the mass flow rate as a function of the vibration amplitude. The experimental results showed that the mechanofusion treatment gave a remarkable improvement in the powder flowability evaluated by the two factors mentioned above. The general tendency of the improvement can be explained by the projected-area ratio defined as the ratio of the total projected-area of nano-particles adhering to core-particle surface and the projected-area of the core-particle itself.