Nowadays, many soil improvement methods have evolved with different materials used. In this study, a series of laboratory tests are carried out to investigate the effect of a polypropylene fiber material on the shear strength and California Bearing Capacity (CBR) characteristics of high plasticity Ankara clay. First, geotechnical properties of Ankara clay are presented. Then, results of unconfined compressive strength, triaxial strength (CU type) and CBR tests conducted on the various polypropylene fiber/clay mixtures (0.1%, 0.2%, 0.3% and 0.4% by dry weight) are given. Samples were compacted at optimum moisture contents and maximum dry unit weights obtained from the standard Proctor compaction effort. Standard Proctor compaction tests and CBR tests revealed that percentage of 19 mm length F type polypropylene fiber did not influence much the optimum moisture content, maximum dry unit weight and CBR values. Unconfined compression tests showed that inclusion of fiber content increased the axial strain at failure. Moreover, consolidated-undrained (CU) triaxial test results exhibited that due to inclusion of 19 mm length F type polypropylene fiber, the cohesion intercept decreased slightly but the internal angle of friction increased considerably. © 2009 Taylor & Francis Group, London.