There is disclosed an impact modifier which comprises a graft copolymer comprising a rubber polymer (A) obtained from a mixture of one or more kinds of (meth)acrylic monomers as major ingredients, having a glass transfer temperature of 0° C. or lower, and forming particles having each of 80 wt % or more of them has a particle diameter within the range of 400 to 1,000 nm as determined from a particle diameter distribution on a weight basis, and branches (B) grafted thereto obtained from one or more kinds of vinyl monomers; a process for producing thereof; and a thermoplastic resin composition containing the impact modifier.