Abstract: Fiber as a good reinforcement material was mainly applied to cement and concrete,however,very limited information has been reported on the use of randomly distributed discrete fibers for soil reinforcement.In this paper,an experimental program was reported to study the individual and combined effects of randomly oriented fiber and cement additives on the geotechnical characteristics of clay soil.Three percentages of fiber contents,i.e.(0.05)%,0.15% and 0.25% by weight of raw soil and two percentages pf cement contents,i.e.,5% and 8% were randomly included in to the soil.Twelve samples were prepared and subjected to unconfined compression tests.The results of this study clearly show that both the polypropylene fiber and cement can improve the strength and stability of raw soil.Fiber inclusion can change the brittle behavior of cement soil to the ductile behavior.The increase in the strength caused by the combined action of cement and fiber is more than the sum of the increase caused by each of them individually.Polypropylene fibers are characterized in terms of their mechanical properties and microstructures.The microstructures were obtained from scanning electron microscopy(SEM) analysis.It is found that there are interfacial interactions between fibers and other substance(such as the hydrations of cement,minerals of clay soil),which can produce significant effects on the bonding strength and friction between the reinforcement and the soil matrix.