Thermoplastic resin compositions

A thermoplastic resin composition which comprises

• (I) 90 to 50% by weight of a propylene polymer other than (III), and
• (II) 10 to 50% by weight of polystyrene other than (III), and
• (III) 5 to 20 parts by weight, based on 100 parts by weight of the aforesaid resins (I)+(II), of a preformed compatibility agent which is a multiphase structure thermoplastic resin which is a graft copolymer comprising 20 to 90% by weight of a propylene polymer and 80 to 10% by weight of polystyrene or copolymer of styrene and acrylonitrile, one of the components being formed as a dispersion phase having a particle diameter of 0.001 to 10 .mu.m, said graft copolymer being obtained by melting and kneading a graft polymerization precursor (A) which is prepared by copolymerizing styrene or styrene and acrylonitrile with at least one of radical polymerizable peroxides having the formulae (a) or (b) ##STR4## wherein R.sub.1 is a hydrogen atom or an alkyl group having 1 or 2 carbon atoms, each of R.sub.2 and R.sub.7 is a hydrogen atom or a methyl group, R.sub.6 is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each of R.sub.3, R.sub.4, R.sub.8 and R.sub.9 is an alkyl group having 1 to 4 carbon atoms, each of R.sub.5 and R.sub.10 is an alkyl group having 1 to 12 carbon atoms, a phenyl group, an alkyl-substituted phenyl group or a cycloalkyl group having 3 to 12 carbon atoms, m is 1 or 2, and n is 0, 1 or 2,
• in particles of a propylene polymer.