Polypropylene (PP) fiber is considered as an innovative material to enhance the strength parameters of soils under both dynamic and static loading conditions. This study is an attempt to improve the liquefaction resistance of loose state sands, which are prone to liquefaction, by the addition of PP fibers as reinforcement. A series of undrained cyclic triaxial tests with various initially constant and subsequent cyclic shear stresses were applied on the saturated sand having a low relative density ranging from 24 to 40%. Tests were carried out on two different sand samples obtained from two different sites, i.e. Topaktas (Van) and Akpinar (Istanbul), using cyclic triaxial test apparatus available in the laboratory. The first one was liquefied on the site during the MW 7.1 Van Earthquake on 23rd October 2011. Polypropylene fibers were used in two different combinations of randomly distributed and layered in the second type of sand. In the case of fibers, the percentages of fibers by weight of dry sand were taken as 0.1 and 1%, respectively. The liquefaction parameters, such as pore water pressure (PWP) and shear stress, have been measured in cyclic tests at a constant frequency of 0.1 Hz. Large strain level deformations and liquefaction phenomena started to occur in the first few cycles of cyclic triaxial tests of unreinforced samples. Moreover, the results of this study indicate that increasing the percentage of fiber inclusion decreases the initial elasticity modulus, Emax; in contrast, fiber addition makes loose sand more resistant against liquefaction under dynamic loading.