This study established the optimum conditions for structural synthetic polymer fiber manufacture to make a fiber having excellent tensile strength. Two polypropylene resins, differing in their melt indices, and four different draw ratio values were investigated, that is, 6, 7, 8, and 9, to establish which had the greatest influence on tensile strength. The tensile strength and the modulus of elasticity were measured for the manufactured fibers. In addition, nanoclay was added to polypropylene and polyolefin fibers to improve the mechanical properties of fiber-reinforced cementitious composites. The optimum mixing ratio and draw ratio of the synthetic polymer fibers were established prior to nanoclay addition. Fiber mixing ratios (polypropylene:polyethylene) of 100:0, 90:10, and 85:15, a draw ratio of 8, and nanoclay additions of 1, 3, and 5 wt% were studied. The modulus of elasticity improved with nanoclay addition, while the tensile strength remained almost unchanged, except in the case of 100% polypropylene with 5 wt% nanoclay. POLYM. COMPOS., 34:1698-1709, 2013. (c) 2013 Society of Plastics Engineers