Abstarct: The soil shear strength parameters have important role in the stability analysis of soil excavations and slopes. Saturated shear strength parameters are generally used in the analysis. The portion of the soil profile above the groundwater table where the pore water pressure are negative is usually ignored. The main objective of this study is to investigating the effect of negative pore water pressure on calculating the factor of safety in unsaturated zones. A computer software program, Geo Studio 2012, was used in order to determining the factor of safety using the extended Mohr-Coulomb failure criteria. Both linear and nonlinear forms of the unsaturated shear strength equation will be used in the study of the effect of unsaturated shear strength on the computed factor of safety. Variation with time of the factor of safety was determined and the effect of variation of permeability with matric suction and the initial depth of the groundwater table in the shape of the Fs-time curve is examined in a parametric study and the effect of shear strength parameters, in calculating the factor of safety, was evaluated and also was demonstrated; considering the low percentage of hydrostatic distribution matric suction would be significantly increase the factor of safety, illustrating the benefit of considering matric suction in the stability analysis. According to the modeling results, unsaturated parameters have significant effect to increasing the factor of safety. The value of factor of safety in unsaturated state depends on shear strength parameters, spetial angle of internal friction. The shape of the Fs-time curve depends largely on the saturated value of soil permeability and the initial depth of the groundwater table. The variation of permeability with matric suction did not significantly affect the analysis results, and the factor of safety depending on the saturated value of soil permeability.