Abstract: This paper discucces some current strength criteria for composites.By the use of the stress tensor invariant principle a general strength criterion of the sacond-order power for three-dimensional unidirectional composites is propsed as following: Consdering the basie failure modes of fiber-reinforced composites,a new strength criterion can be derived in the form: where σ_X is longitudinal stress and σ_Y and σ_Z are transverse stresses.