The critical failure surface for a given slope can be determined by comparing factor of safety of several trial slip surfaces. To find the minimum factor of safety, it is important to get critical failure surface for the given slope. Different searching and optimization methods that had the difficulty in using them for hand calculations have been used in the past. In this study, effect of soil strength parameters; cohesion (c), internal friction angle (ϕ), and unit weight () on the failure surface and factor of safety of the slope were studied. GEO5 software program were used to calculate the length of failure and determine the critical failure surface. An equation was introduced in to locate the critical failure surface by using soils strength and slope geometry parameters. The results of the study showed that the factor of safety of the slope changes with varying cohesion c, internal friction angle ϕ, and the unit weight  of the soil. Moreover, the slip surface is affected by the dimensionless function (), which is related to the cohesion, internal friction angle and unit weight. Model was introduced in to locate the critical failure surface by using soils strength and slope geometry.