An experimental program was undertaken to investigate the effects of discrete polypropylene fibers and Class C fly ash on the stress-strain and shear strength behavior of clayey soil. Two types of fiber (fibrillated polypropylene fiber and multifilament polypropylene fiber) in two different lengths (6.0 mm and 19.0 mm) and two fiber dosages (i.e. 0.5% and 1.0% by dry weight of soil) were considered. At first, compaction characteristics of the untreated soil and seven different fly ash-soil mixtures (2.5%, 5.0%, 7.5%, 10.0%, 15.0%, 20.0%, and 30.0% by dry weight of soil) were evaluated at standard compaction energy. Considering compaction characteristics, unconfined compression tests of untreated soil and five different fly ash-soil mixtures (5.0%, 10.0%, 15.0%, 20.0%, and 30.0% by dry weight of soil) were carried out after 1-, 7-, 14-, 28- and 90-day curing periods. According to the obtained test results, untreated soil and two different fly ash-soil mixtures (i.e. 10.0, and 30.0% by dry weight of soil) were mixed with fibrillated polypropylene fiber and multifilament polypropylene fiber in two different lengths and in two different fiber dosages, separately. A total of 17 different fiber-fly ash-soil mixtures were composed, and their unconfined compression tests were carried out after 28-day curing period. According to unconfined compression test results, 6 different fiber-fly ash-soil mixtures were selected for UU triaxial tests at the end of 28-day curing period. The obtained test results indicated that the effect of fly ash content on the stress-strain behavior is superior to the effect of fibers. The post-peak strength behavior is strongly affected by fiber type, length and dosage. The magnitude of the effect increases as the fly ash content increases. Inclusion of fiber alone without fly ash decreases unconfined compressive strength (UCS) of the compacted fiber-clay mixtures. On the other hand, when combined with fly ash, fiber inclusion increases UCS depending on fiber type, length and dosage. Moreover, for both fiber types and fiber lengths, the higher the dosage of fiber, the higher the UCS is. The increase in UCS becomes as much as 218% for 30% fly ash content and 1.0% 19 mm long fibrillated polypropylene fiber. (C) 2015 Elsevier B.V. All rights reserved.