The generation of complex nanostructures to obtain novel characteristics and improved performance has been achieved by coupling multiple nanoscale reactions. Because reactions at the nanometer scale directly govern the morphology of nanostructures, understanding the reaction mechanism is critical to precisely control the morphology and, eventually, the physicochemical properties of the materials. However, because of the ensemble-average effect, investigating the reaction mechanism at the bulk level does not provide sufficient information. In this study, we investigated the overall sulfidation reaction mechanism that occurred on individual silver nanocubes in real time at high temperature. Using the single-particle dark-field imaging technique, three discrete steps of the sulfidation reaction were clearly resolved in the profiles of the plasmon peak shift and the intensity change of individual particles according to time progress: (I) reactant diffusion to the silver surface by passing through a ligand barrier, (II) silver sulfide formation by C-S bond cleavage of cysteine molecules, and (III) diffusion of silver atoms in the silver sulfide layer until the complete formation of silver sulfide. By a combination of simulation and control experiments, physical constants were derived for each step, which is completely hidden in the ensemble measurements. Each individual nanoparticle exhibited a large variation of physical values, such as the reaction rate constant and diffusivity, mainly resulting from the intrinsic structural heterogeneity. Dark-field microscopy image processing based on surface plasmon scattering would be helpful to analyze the reaction kinetics and understand the reaction mechanisms of the numerous multistep nanoscale reactions in real time with high spatial and temporal resolutions under actual reaction conditions.