A resin composition comprises 1 to 60 parts by weight of a wollastonite fiber (in which the number-average fiber length is 1 to 30 .mu.m, the number-average fiber diameter is 0.1 to 10 .mu.m, and the proportion of the number of a fiber having a fiber length of 5 to 25 .mu.m relative to the total number of the fiber is 20 to 75%) and 0.1 to 20 parts by weight of an acid- or epoxy-modified olefinic resin having a melting point of 100 to 170.degree. C., relative to 100 parts by weight of a thermoplastic resin (e.g., a polycarbonate-series resin). The weight-average molecular weight of the modified olefinic resin may be about 2.5.times.10.sup.4 to 30.times.10.sup.4. The resin composition may comprise a flame retardant. The combination use of the wollastonite fiber and the modified olefinic resin improves heat resistance, impact resistance and weld strength, even in the case of using a wollastonite fiber having a number-average fiber diameter of about 4 to 7 .mu.m and an average aspect ratio of about 1.5 to 3. The present invention provides a thermoplastic resin composition for improving the impact resistance, weld strength and surface property without deteriorating the heat resistance.