A process for preparing an enlarged latex, which comprises (1) causing (a) an anionic surfactant and (b) at least one surfactant selected from the group consisting of a cationic surfactant and an amphoteric surfactant to exist in a latex, (2) adding, as an aggregating and enlarging agent, at least one selected from the group consisting of (i) an inorganic acid, (ii) an organic acid, (iii) a substance which forms an acid in water, (iv) a combination of at least two substances which are reacted with each other to form an acid, and (v) a substance which forms an acid by exposure to active rays to the latex in the presence of these surfactants, and (3) causing the acid derived from the aggregating and enlarging agent to act on the latex, thereby enlarging the particle diameter of the latex, an enlarged latex obtained by the preparation process, a graft copolymer obtained by polymerizing a polymerizable monomer with the enlarged latex, and a resin composition comprising the graft copolymer and a thermoplastic resin.