The use of soil-fiber has been recognized as a viable technique of soil improvement in numerous geotechnical applications. However, existing research has not yet established the fundamental mechanisms controlling the behavior of clay-fiber mixtures and the constraints that may affect their performance. On the other hand, there is a growing increase in the development of methodologies for predicting soil-fiber behavior, as well as the use of soil-mixture constitutive models. In this context, the present work seeks to contribute to the understanding of shear strength and deformability behavior of a clay soil reinforced with short polypropylene fibers (0.1 and 0.25%) under drained and undrained triaxial tests. In terms of effective stresses, in drained condition, results showed that the fibers caused an increase in soil cohesion, remaining the angle of friction practically unaffected. In undrained condition, in terms of effective stresses, fibers did not affected the soil cohesion and an increase in the friction angle was observed. Drainage during shearing have favored soil-fiber shear strength. The pore water pressures during shearing (undrained condition) could have caused perturbation and rearrangement at the soil-fiber interface, thus weakening the interfacial resistance between the soil and the fibers. Results evidenced the need to evaluate the soil-fiber in field drainage conditions.