Method for manufacturing structural synthetic fibers for concretes and mortars and the synthetic fiber obtained, particularly a polyolefin-based synthetic fiber for use in concretes and mortars with a hydraulic binder as a cementing base for increasing the elastic modulus of the same which comprises the mixture of polyolefin and additives, the formation of threads, a first stretching process, an alternate series of heating-cooling, collecting the threads in coils and stabilization of the polymerization/cross-linking for two days to one week; the threads are subsequently again subjected to one or several re-stretching phases, including the heating-cooling and subsequent stabilization processes if successive re-stretching processes are going to be carried out; lastly a surface etching phase and the cutting of the threads.