The Improvement of problematic soils has long been regarded as a main concern by civil engineers. Dune sands are considered as main common problematic soils due to their uniform gradation and lack of cohesion. So far various methods have been used for the soil modification. In this research, a combination of casein and polypropylene fibers has been used to improve the strength parameters of a typical dune sand. To prepare the stabilized soil samples, resin amounts of 1, 1.5 and 2%, and 12 mm length fibers of 0.05, 0.1 and 0.15%, relative to dry weight of the soil were used. Direct shear tests were employed at three levels of normal stresses of 50, 100 and 150kPa to determine the strength parameters of the stabilized samples provided with a treatment time of 3 days. The results of the experiments showed that increasing the amount of casein resin increases the soil strength. Adding the fiber up to 0.1% caused more strength enhancement. Thus, under a normal stress of 100kPa, the shear strength of pure sand sample as well as samples stabilized with 1, 1.5 and 2% of the resin in combination with 0.1% fibers were 67.75, 107.53, 329.82 and 474.75kPa respectively. It is observed that the shear strength has increased by increasing the amount of resin, so that at the additive content of 2% resin and 0.1% fiber the shear strength has reached to 5.8 times of the pure sand strength. The effect of wetting on the strength of the stabilized samples was also investigated. As the strength of the stabilized samples decreased under wetting conditions, small amounts of hydrated lime was used to maintain the strength of the samples in the presence of water. The results showed that the addition of lime increases the resistance of samples under wetting conditions. The optimum amount of lime required to maintain the sample strength was determined to be about 0.5% of dry weight of the soil samples. The CBR standard tests have also been used to investigate the performance of the stabilized soil as a subgrade layer. In this context, the CBR tests were conducted on the samples stabilized with 2% casein resin and 0.1% fibers as optimum additive mixture under both dry and saturated conditions. Similar tests were also conducted on samples stabilized with 2% resin as well as pure sand samples. In dry conditions, the specimens were cured at laboratory environment for 3 days. To achieve saturation conditions, the samples were immersed in a water sink for 4 days. The highest CBR values of 34% and 22% were obtained for samples with optimum additive content respectively for dry and saturated conditions. It is observed that even in saturation conditions the CBR value of the soil stabilized by 2% resin and 0.1% fiber is close to the CBR value of the soil stabilized by 2% resin in dry conditions i.e. 23%. Finally, scanning electron microscopy (SEM) images were used to determine how to improve the bond between particles Key words: dune sands, casein resin, polypropylene fibers, direct shear test, California Bearing Ratio tests, electron microscopy images