An Atomic Force Microscope (AFM) was used to analyze the surface of a lead plate when the reaction corresponding to what occurs at the negative electrode of a lead acid battery, Pb + SO42- reversible arrow PbSO4 + 2e, took place. At the beginning, when the lead plate, on the surface of which lead oxide already existed, was in contact with sulfuric acid, lead sulfate crystals were formed and gradually grew with time without applying any potential. In situ AFM observation of the formation and the growth of lead sulfate at the potential corresponding to the discharge reaction was attempted. And it was possible along with the reduction of the lead sulfate, which formed in discharging process. The results also showed that the chemically formed lead sulfate crystals were clearly different in appearance from the electrochemically formed ones. These results were also applied to a study of the mechanism of the sulfation process that often occurs in the active material of the negative electrode of a lead acid battery.