Abstract: Complementarywelastic enemy derive is used to derive a general nonlinear constitutive relation of unidirectional composite laminae under plane stress.Absence of the norlinear coupling between longitudinal and transverse stresses for unidirectional composite iamtnae is shown by aniaxial tension tests (in longitudinal direction and tranaverse) and biaxiai tension tests, and a simplified and practical constitutive relation is obtained.An off-axial tension test is used to prove the present theory valid.