Micrometer-sized, monodisperse, "mushroom-like" Janus poly(methyl methacrylate)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate)-graft-poly(2-(dimethyl amino)ethyl methacrylate) (PMMA/P(S-BIEM)-g-PDM) particles were successfully synthesized by site-selective surface-initiated activator generated by electron transfer for atom transfer radical polymerization in aqueous dispersed systems with spherical PMMA/P(S-BIEM) composite particles having controlled morphologies prepared using the solvent evaporation method. The anisotropic nonspherical shape of the obtained particles was controlled by changing the percentage of the surface area occupied by localized initiation sites (bromine group) at the surface of the PMMA/P(S-BIEM) composite particles with different P(S-BIEM) contents. Grafted PDM layer formed at the surface (contacting with water) of the P(S-BIEM) phase reversibly exhibited the volume phase transition in response to temperature and pH, which gave different nonspherical shapes ("open" or "closed" mushroom-cap). On the basis of such dual stimuli-responsive properties, the nonspherical particles effectively operated as particulate surfactant for Pickering emulsion, resulting in a stable 1-octanol-in-water emulsion at optimum temperature and pH value, and the Pickering emulsion could be easily unstabilized quickly by controlling them.