Monotonic triaxial compression, triaxial extension and torsional shear tests were carried out on geotextile reinforced sand and reinforced clay, mainly to investigate the effects of rotation of principal stresses on the mechanical behavior of the reinforced soil materials. The tests were carried out on unreinforced and reinforced specimens with 2, 3 and 4 geotextile layers under three different confining pressures. Investigation of the monotonic behavior of the reinforced materials under different stress paths, i.e. triaxial compression, triaxial extension, and torsional shear shows that direction of principal stresses can have profound effects on the stress-strain curve, shear strength, and slope and intercept of failure envelope. Test results reveal that geotextiles improve the mechanical properties of the sand and clay, since both strain at failure and undrained shear strength increase with the number of geotextile layers in sand and clay. In addition, test results indicate that geotextile inclusion enhances the mechanical properties of geotextile reinforced sand and clay, however, geotextiles seems to be more effective when used to reinforce sands.