AbstractIn this note, a new method to calculate the equivalent Mohr–Coulomb friction angle ϕ′mc for cohesive and frictional materials is presented. This method makes a transformation from the failure surface for cohesive materials to the failure surface for cohesionless materials and obtains ϕ′mc as well as the principal stress ratio σ′1/σ′3 for cohesionless materials in the transformed space first, then obtains ϕ′mc for cohesive materials by linking σ′1/σ′3 in the transformed space and in the original space. In the application example, an analytical solution of the invariant stress ratio L is derived from the failure function in the transformed space. The influence of the intermediate effective principal stress σ′2 is also demonstrated using the already calculated ϕ′mc. Copyright © 2010 John Wiley & Sons, Ltd.