The purpose of this note is to propose a relation somewhat similar to the size-strength relationship of brittle materials failing in tension in accordance with the Weibull theory. According to the failure theory of composites, a plot of the natural log of failure stress (ln σf) versus the natural log of volume (ln V) for a uniaxially reinforced composite is a curve that is concave upward. A simple analytical relation representing this concave failure curve could be very useful, since current methods of constructing the line are quite complex and necessitate the adoption of debatable assumptions about the magnitudes of a number of the many parameters that appear in the theory. The relation proposed in this note gives the theoretical failure stress of uniaxial composites to a surprising degree of accuracy.