Abstarct: Nowadays, geotechnical studies are very important in factories dealing with chemicals. Industrial effluents contain different chemicals that penetrate the soil and change the structure of the soil. The aim of this study is to investigate the shear strength parameters of the Jajarm Alumina Factory bed soil contaminated with sodium hydroxide. Different concentrations of NaOH were prepared in 1.5 to 12 weight percent, and the direct shear test was conducted in two separate phases. The first direct shear test was conducted on uncontaminated dry and saturated soils and with different percentages of NaOH contamination (1.5 to 12 weight percent). The results of the first phase showed that the soil cohesion significantly increased with the increase in NaOH concentration from 0 to 12% so that it increased from 0 to 25.96kPa as a result of chemical reactions of NaOH in fine-grains of soil and the formation of salt crystals in coarse-grains leading to higher cohesion of soil particles. The internal friction angle of soil reduced from 33.75 to 28.30. The second phase of the direct shear test was conducted on soil samples with 3, 6, and 9% NaOH contamination and with the effect of aging in (0, 7, 14 & 28) days. The results of the second phase indicated that by increasing the concentration of NaOH in all time intervals, cohesion decreased and soil internal friction angle increased. It is noteworthy that by increasing curing time from 0 to 28 days at a determined percentage, cohesion and internal friction angle increased significantly.