This research studies the effect of the addition of polypropylene fibers (PF) on the properties of the swelling clayey soil. The effect of PF on the shear strength of the clayey soil has been studied and the results show that adding PF, up to specific value, increases the shear strength. Beyond this value of addition, the shear strength decreases. The increase in shear strength was about (100-110) % and the optimal addition ratio was determined.