Abstarct: All direct and indirect factors should be considered for designing rock or soil slopes either in mines or dames or road trenches. Weathering and alteration as destructive agents strongly influenced rock geomechanical parameters and reduce the stability and strength of rock considerably. Schist of Gol-ohar iron mine were strongly affected by alteration and weathering. Up to now, these parameters have been studied for stability analysis from a qualitative point of view and geology. However, studying the effect of these factors on geomechanical parameters is impossible except through numerical and quantitative methods. In the current thesis, enormous quantitative data were collected for investigating the effect of weathering and alteration on geomechanical parameters of rocks in mine#1 of Gol-Gohar. Various degrees of weathering and alteration of rocks were indicated by desert indices and their physical and mechanical characteristics were studied in laboratory. The amount of weathering and alteration degree of attenuation factor was indicated for each parameter through the slope stability probability classification (SSPC). The results show that most of geomechanical parameters of rocks are dramatically reduced by increment of weathering and alteration degree. Uniaxial compressive strength, tensile strength and static modulus of elasticity are affected by weathering more than other characteristics. The results of experiments uncover that the schist’s vulnerability of Gol-Gohar iron ore mine from weathering reaches to its maximal value after grade III. In addition, controlling the inclined surfaces is an issue of geotechnical issues, which has high percentage of uncertainty. The effect of uncertainty on prediction of slopes stability is usually important. In deterministic methods, stability analysis is mostly done baxsed on Safety factor; however, these methods are unable to clarify the effect of uncertainties. Probabilistic stability analysis of inclined surfaces is a suitable tool for considering the effect of uncertainties in design processes. In the current research various uncertainties in studying the geotechnical parameters and fundamentals of common probabilistic methods such as Mont Carlo simulation as a probabilistic method for reduction of uncertainties of geotechnical parameters that recently widely used, were introduced. In addition, through Slide Software program and selecting the Bishop limit balance as performance function, Monte Carlo probabilistic analysis was implemented on walls of Iron mine #1 of Gol-Gohar and effect of weathering and alteration on amount of Safety factor was studied. After quantifying the effects of weathering and alteration it was clarified that the Safety factor of degree IV reduces up to 0.74 of its primary value.