Groundwater seepage flow is one of the main driving factors that may cause global instability for geosynthetically-reinforced slopes. This study investigated the use of synthetic fibers against seepage forces since they are known to increase the shear strength of soils. For this purpose, 100 cm long, 20 cm wide, 55 cm high physical slope models with 45° slope face were constructed by compacting 2.5 cm thick lifts in a Plexiglas soil box. The hydraulic gradients in these models were applied with three different hydraulic heads (25 cm-, 38 cm- and 50 cm-H2O) boundary conditions (BC) which were kept constant during the tests. The pore water pressures developed in the slope were measured by 22 pencil size tensiometers placed on one side of the soil box. Two different soils were used: 1: Bare sand (no fiber inclusion), and 2: sand with 18 mm long polypropylene (PP) fiber inclusion. As a result of soil erosion under 25 cm-H2O BC, a shallow-seated failure surface starting from the slope face and exiting at the toe was observed in the bare slope experiment. Deep-seated failure surfaces starting from the crest of the slope and exiting at the toe were observed in the bare slope experiments conducted under both 38 cm- and 50 cm-H2O boundary conditions. On the other hand, it was observed that PP fiber inclusion prevented seepage erosion and progressive slope failures of that bare slope experiments under the same hydraulic head BCs as the apparent effective cohesion value for sand treated with 18 mm PP fiber with gravimetric fiber content of 1% reached to 31 kPa, which was obtained from consolidated-drained (CD) triaxial shear strength test.