Electrochemical hydrogenation/dehydrogenation properties were studied for a single particle of a AB5-type Mm-based (Mm: misch metal) hydrogen storage alloy, MmNi3.55Co0.75Mn0.4Al0.3. A carbon fiber microelectrode was manipulated to make electrical contact with an alloy particle, and the cyclic voltammetry and the galvanostatic charge/discharge experiments were performed. A single particle of the alloy showed the discharge capacity of 280 mAhg-1, the value being 90% of the theoretical capacity. In addition, the hydrogenation and the anodic oxidation of Co element in the alloy were clearly separated to be able to discuss in detail. Data were compared with that of the composite film consisting of the alloy particles and a polymer binder, which is more practical form for Ni-MH batteries. The potential-step experiment was also carried out to determine the apparent chemical diffusion coefficient of hydrogen atom (Dapp) in the alloy. Since the alloy particle we used here was a dense, conductive sphere, the spherical diffusion model was employed for data analysis. Dapp was found to vary the order between 10-9 and 10-10 cm2 s-1 over the course of hydrogenation/dehydrogenation process.