Abstract: It is observed that randomly distributed polypropylene fibers act to interlock particles and thus improve the mechanical behavior of the reinforced soils. The polypropylene fiber inclusion clearly increases the shear strength of soils, and reduces the loss of post-peak strength. The stress-dilatancy relationship can be employed as the foundation to develop a constitutive model for polypropylene fiber-reinforced (PFR) soils. In this study, a number of conventional triaxial compression tests are carried out to investigate the effect of randomly distributed fiber reinforcements on the stress-dilatancy relationship of Nanjing sand. A new parameter, \sigma_FR, representing the reinforcing effect of fibers in Mohr-circle space is introduced to describe the behaviors of PFR sand. When a polypropylene fiber-soil assembly dilates in response to the applied shear deformations, the work done by the driving stress will be dissipated by the particle sliding and the fiber deformation. The work dissipated by the polypropylene fiber deformation can be expressed by introducing \sigma_FR. Based on the minimum rate of internal work assumption, the stress-dilatancy relationship is deduced for fiber-reinforced sand. It is shown that the predicted results are in good agreement with the experimental ones.