This paper investigates the effect of recycled polyester fiber, produced from polyethylene(PET) bottles, in combination with nano-Si O2 as a new stabilizer to improve the mechanical properties of soils. We intend to study the effect of adding nano-Si O2 and recycled polyester fiber on soil engineering properties,especially the shear strength and unconfined compressive strength(UCS), using clayey soil with low liquid limit. Three different combinations of fiber-soil ratios ranging between 0.1% and 0.5% as well as three different combinations of nano-soil ratios ranging between 0.5% and 1% are used. The shear strength and UCS of treated specimens are obtained from direct shear test and unconfined compression test, respectively. Results of this study show that the addition of recycled polyester fiber and nano-Si O2 increases the strength of soil specimens. Both the shear strength and UCS are improved by increasing the contents of recycled polyester fiber and nano-Si O2 in the soil mixture. The increase in the nano-Si O2 content leads to a reduction in failure strain, but the increase in the content of recycled polyester fiber leads to an increase in failure strain. The increase in the contents of recycled polyester fiber and nanoSi O2 leads to an increase in elastic modulus of soils. Based on the test results, the addition of recycled polyester fiber improves the mechanical properties of soils stabilized with nano-Si O2 as well as the recycled polyester fiber has a positive effect on soil behaviors. This paper investigates the effect of recycled polyester fiber, produced from polyethylene (PET) bottles, in combination with nano-Si O2 as a new stabilizer to improve the mechanical properties of soils. We intend to study the effect of adding nano-Si O2 and recycled polyester fiber on soil engineering properties, especially the shear strength and unconfined compressive strength (UCS), using clayey soil with low liquid limit. Three different combinations of fiber-soil ratios ranging between 0.1% and 0.5% as well as three different combinations of The shear strength and UCS of treated specimens were obtained from direct shear test and unconfined compression test, respectively. Results of this study show that the addition of recycled polyester fiber and nano- Si O2 increases the strength of soil specimens. Both the shear strength and UCS are improved by increasing the contents of recycled polyester fiber and nano-Si O2 in the soil mixture . The increase in the nano-Si O2 content leads to a reduction in failure strain, but the increase in the content in recycled polyester fiber leads to an increase in failure strain. The increase in the contents of recycled polyester fiber and nanoSi O2 leads to an increase in elastic modulus of soils. Based on the test results, the addition of recycled polyester fiber improves the mechanical properties of soils stabilized with nano-Si O2 as well as the recycled polyester fiber has a positive effect on soil behaviors.