Abstract: A high-strength and highly wet-heat-resistant polyvinyl-alcohol-based fiber--in which the crosslinking agent has hardly been oxidized by the heat at the drawing time upon preparation of the fiber, the crosslinking agent has not exhaled so much at the time of dry heat drawing, and the crosslinking agent has penetrated even inside of the fiber so that not only the surface but also the inside of the fiber has sufficiently been crosslinked--can be obtained by the steps of: preparing a polyvinyl-alcohl-based fiber by spinning the polyvinyl-alcohol-based solution, wet drawing the fiber, applying an acetalization compound of an aliphatic dialdehyde having at least 6 carbon atoms to the fiber, subjecting the fiber which contains above compound to dry heat drawing to a total draw ratio of at least 15, and then crosslinking the drawn filament with an acid under mild crosslinking treatment conditions.

Abstract: A synthetic fiber suited for use as a reinforcement for cement mortar or concrete as derived from monofilaments having a tensile strength of not less than 80 kg/mm.sup.2, Young's modulus of not less than 2,300 kg/mm.sup.2, a flat cross section in which the minor axis-to-major axis ratio is 1:2 to 1:10, and a fineness of 1,000-9,000 denier, and being cut to a fiber length such that the aspect ratio (the value obtained by dividing the fiber length by the diameter of a circle equivalent in area to the cross section of the fiber) amounts to 20-150, in particular a polyvinyl alcohol fiber having the above-mentioned characteristic features, shows good dispersibility in cement mortar and produces very good reinforcing effect. Materials of construction which are excellent in workability and postcure strength can be obtained by incorporating the above-mentioned fiber to cement and the like.