The tensile properties of poly(propylene) (PP) composites loaded with multi-walled carbon nanotubes (MWCNTs) were measured using a universal material property testing machine at a tensile speed of 20 mm/min to investigate the effects of the inclusion scale and concentration on the tensile behavior. The Young’s modulus and the tensile strength of the composites were greater than those of the neat PP; with raising the inclusion concentration, they were enhanced slightly except for individual data points, while the tensile break elongation almost decreased; the influence of the filler scale on the tensile behavior was insignificant. At lower tension rate, the reinforced mechanisms of the MWCNT in the PP composites were explained. Furthermore, the relative tensile yield strength was calculated using an equation previously published; fine agreement between the calculated relative tensile yield strength and the measured results was seen from the experiments.