Thus many attempts have been made to use these materials for improving soil engineering properties and so reduce the environmental pollutions. Polypropylene fibers (PP fiber) and worn tires are two types of the waste materials which attracted increasing attentions in soil reinforcement usages. Nevertheless the mixed effects of these materials on the soil properties have not been discussed till now. The main aim of this research study is to investigate the strength parameters of sandy and silty sand soils reinforced with tire chips and PP fibers both in individually and combined percentages using direct shear and CBR tests. In this study, tire chips with 24 mm length, 6 mm width, 5 mm thickness, and aspect ratio of 4 mixed with the soil samples in contents of 15%, 18%, 21% and 25% by weight of dry soil. The PP fibers were creamy colour fibers with random lengths produced as waste materials of carpet factories. Thus the effects of their lengths could not be considered in this investigation. These fibers were also added and mixed with soil samples in weight percentages of 0.1%, 0.2%, 0.3%, 0.4%, 0.6% and 0.8% for conducting different strength tests.Direct shear tests on the sandy samples as well as silty sand samples reinforced with PP fibers showed that the reinforcement makes an increase in the shear strength, internal friction angle and cohesion of the soil samples. However, the amount of increase in the soil cohesion was more pronounced in comparison with internal friction angle. Based on the obtained results, the optimum percentage of fibers for achieving maximum strength was 0.4% for the sand and 0.3% for the silty sand. The direct shear tests on sandy samples reinforced with rubber chips showed results similar to those of fibers i.e. the soil cohesion was most affected by the amount of chips. Nevertheless, adding the chips to silty sandy soil, was most effective on the soil internal friction angle. The optimum percentage of chips for achieving maximum strength was 21% for the sand and 25% for the silty sand. Combining the sandy soil samples with the fiber content of 0.4% and various amounts of tire chips, it was found that the chips content of 21% had the most increasing effect on the strength of the sandy samples with combined reinforcement of fibers and chips. Also, mixing the sandy soil samples with the chips content of 21% and various amounts of fibers, it was observed that the fibers content of 0.3% had the most enhancing effect on the strength of the sandy samples with combined reinforcement of chips and fibers. Moreover, in the silty sandy soil samples mixed with a combination of optimum percentage chips (25%) and different contents of fibers, it was found that the fibers of 0.1% was the most effective content on the strength enhancement of the silty sand samples with combined reinforcement of chips and fibers.