ABSTRACT Investigated from a fundamental point of view is the strength of granular material in simple shear. The theoretical relation between the angle ϕs defined by tan ϕs = τ/σ and the angle ϕp defined by sin ϕp = (σ1 − σ2)/( σ1 + σ3) presents a useful information for engineering purpose. The theoretically derived relationship between the direction of major principal axis to the vertical ϕ and tan ϕs made clear that the expression of tan ϕs = κ tan ϕ (κ is a constant) is an approximate relation. The direction angle of the slip plane to the horizontal was also theoretically obtained. This angle is related to the difference between ϕs and ϕp and the slip plane never coincides with the horizontal at any instance. The relationship between ϕp and ϕs based on the author’s S model yields more realistic result than that based on the sliding block model. In this investigation, assumed is that the orientation of principal strain increment axis coincides with that of principal stress axis for the deformational regime after the most compactive point at which the positive dilatancy begins to take place.