An olefin (co)polymer composition of the present invention comprises 99 to 70 wt. % of olefin (co)polymer (I) and 1 to 30 wt. % of polyethylene-based polymer (II). The olefin (co)polymer (I) comprises 0.01 to 5.0 parts by weight of high molecular weight polyethylene (a) having an intrinsic viscosity &eegr;A measured in tetralin at 135° C. of 15 to 100 dl/g, which is an ethylene homopolymer or an ethylene-olefin copolymer comprising at least 50 wt. % of ethylene polymerization units, and 100 parts by weight of olefin (co)polymer (b) other than the high molecular weight polyethylene. The polyethylene-based polymer (II) has an intrinsic viscosity &eegr;II measured in tetralin at 135° C. of 0.10 to 10 dl/g, which is an ethylene homopolymer or an ethylene copolymer comprising at least 50 wt. % of ethylene polymerization units. Thus, the present invention provides a polypropylene-based olefin (co)polymer composition having excellent formability that is suitable for thermoforming such as vacuum forming and pressure forming, blow molding and foam molding, especially suitable for thermoforming.