The paper applies the boundary perturbation method (BPM) to optimal plastic design under bending with considerable shear effects. This method uses expansion of stress components and of the unknown boundary into power series of a small parameter. In the present paper the small parameter a represents the effects of shear. The shape is described by a power series resulting from boundary conditions. The loading of the cantilever beam consists of a concentrated moment and distributed loading regarded as a perturbing factor. The material of the beam is perfectly plastic, subject to the Huber–Mises–Hencky yield condition. The beam is in the plane strain state.