A plastic molded product of olefin (co)polymer composition comprises 0.01-5.0 weight parts high molecular weight polyethylene with an intrinsic viscosity .eta..sub.E in the range of 15-100 dl/g measured in 135.degree. C. tetralin and preferably 100 weight parts polypropylene. The high molecular weight polyethylene has an average particle size in the range of 1-5000 nm and is finely dispersed as fine granules in the polypropylene, so that high melt strength and high crystallization temperature are attained, enabling various molding methods to be performed with high speed productivity. Using the polymer composition, plastics such as a resin foam, a film, a sheet, a layered plastic article with a coated substrate surface, a hollow plastic article, an injection molded plastic article, a fiber, a nonwoven web or a continuous tube-shaped plastic are provided. The olefin (co)polymer composition is obtained by preparing high molecular weight polyethylene granules using e.g. a titanium transition metal compound catalyst, and adding these, e.g. as a polymerization catalyst of propylene, singly or together with a polyolefin preparing catalyst.