A polyethylene article such as film or fiber having high strength and high elastic modulus is produced by orienting an ultra-high molecular weight polyethylene having an intrinsic viscosity at 135.degree. C. in decalin of 5 to 50 dl/g, said ultra-high molecular weight polyethylene being prepared by homo- or copolymerizing ethylene in the presence of a catalyst system and hydrogen, said catalyst system comprising (1) a solid catalyst component containing at least magnesium, titanium and/or vanadium, (2) an organometallic compound and (3) a compound represented by the following general formula �I!: ##STR1## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each hydrogen or a hydrocarbon residue, with the proviso that any two of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 may conjointly form a cyclic hydrocarbon residue.