Abstract: By poststretching, at a temperature between about 135.degree. and 160.degree. C., a polyethylene fiber, which has already been oriented by drawing at a temperature within 5.degree. C. of its melting point, an ultra high modulus, very low creep, low shrink, high tenacity polyolefin fiber having good strength retention at high temperatures is obtained. The poststretching can be in multiple stages and/or with previous annealing. The poststretching should be done at a draw rate of less than 1 second.sup.-1. Tensile modulus values over 2,000 g/d for multifilament yarn are consistently obtained for ultrahigh molecular weight polyethylene, with tensile strength values above 30 g/d while at the same time dramatically improving creep (at 160.degree. F. (71.1.degree. C.) and 39,150 psi load) by values at least 25% lower than fiber which has not been poststretched. Shrinkage is improved to values less than 2.5% of the original length when heated from room temperature to 135.degree. C.