Tensile strength is one of the most im porta nt mechanical properties of structural short fibre composites, and its prediction is essential for composite design. This paper develops a strength theory for three-dimensionally oriented short fibre-reinforced composites. The contribution of direct fibre strengthening to the composite strength is derived using a maximum-load composite failure criterion. Other strengthening mechanisms, such as residual thermal stress, matrix work hardening and short fibre dispersion hardening are also incorporated into the calculation of composite strength. In the derivation of direct fibre strengthening, the strain and stress of short fibres with different inclination angles were first derived, and the direct fibre strengthening was calculated from the maximum total load these short fibres can carry in the composite loading direction.