A plastic bumper for an automotive vehicle, of the type having so-called floating ribs for the purpose of weight-lightening and preventing surface sinkage due to resin shrinkage. The bumper is formed of a composition comprising (a) propylene-ethylene block copolymer in an amount ranging from 51 to 60 weight %, the block copolymer containing a content soluble in xylene at ordinary temperature, the content ranging from 3 to 8 weight %, and ethylene in an amount ranging from 3 to 8 mol %, the block copolymer having a MFR ranging from 20 to 40 g/10 min; (b) ethylene-propylene based copolymer rubber in an amount ranging from 20 to 30 weight %, the copolymer rubber having a MFR ranging from 0.1 to 2 g/10 min; and (c) talc in an amount ranging from 17 to 25 weight %, said talc having an average particle size ranging from 1.0 to 2.5 .mu.m. Additionally, the composition has (I) a flexural elastic modulus (at 23.degree. C.) not lower than 17000 Kg/cm.sup.2, (II) an Izod impact value (at -30.degree. C.) not lower than 6 kg-cm/cm, (III) a coefficient of linear expansion (3 mm thickness, 45 mm length, at 23.degree. to 80.degree. C.) not higher than 7.0.times.10.sup.-5 (cm/cm/.degree.C.), and (IV) a MFR not lower than 7 g/10 min.