Abstract: Fiber-reinforcement is a newly developed soil improvement technology. Better understanding the mechanical behaviors of fiber-reinforced soils is significant for evaluating the stability of the relevant earth structures and extending the application of this technology in engineering. This investigation aims to study the shear strength characteristics of fiber-reinforced unsaturated cohesive soils. Polypropylene fiber is used as the reinforcement material. A series of direct shear tests are performed under controlled water content and dry density conditions. With the application of scanning electron microscope (SEM), the fiber reinforcement mechanisms are discussed, and the obtained macro behaviors are interpreted from micro level. The shear test results show that the inclusion of fiber in soils can significantly enhance the shear strength of soils, which increases with increasing fiber content. The fiber reinforcement benefit on cohesion force is more evident than that on internal friction angle. The shear strength decreases with the increasing water content and increases with the increasing dry density. Moreover, the fiber reinforcement contribution on strength is more pronounced under relative low water content and high dry density conditions, where the fiber reinforcement benefit can be motivated effectively. It is also found that the fiber reinforcement can increase the strain at failure and reduce the peak strength loss after failure and therefore improve the ductility of soil specimens. Based on the SEM analysis, it can be concluded that the 1D reinforcing effect of a single fiber and the 3D reinforcing effect of fiber mesh are the dominant mechanisms of fiber-reinforced soils, which are conditioned by the interfacial mechanical interactions between fiber and soil particles. The SEM images also indicate that fibers on the shear surface may be either pulled out or broken during shear.