Slope failures can cause delay in the production schedule and the loss of life and equipment. In this study, slopes excavated in very weak rock masses where expected failure mode is circular failure have been studied. The effects of slope height, slope angle, water saturation, cohesion, internal friction angle and density on slope stability under circular failure conditions have been investigated by three methods: Hoek and Bray stability diagrams, Bishop's simplified method of slices, and finite difference numerical code, FLAC(3D) (Itasca 1997). Safety factor calculations have been carried out for the various values of parameters and obtained values are compared with each other. However, the main focus is on the results of the numerical modeling. The presence of correlation between the studied parameters and the factors of safety obtained from numerical models has been searched and the fitted equation has been given.