Stiffness reduction in [S,90n]s laminates due to transverse cracking in 90- layers is modeled. Closed form expressions for axial modulus and Poisson’s ratio reduction as the crack density increases are obtained. These expressions are convenient for use and they contain, apart from crack density, geometrical parameters and elastic constants of layers, only the average opening displacement of the transverse crack (COD) that is normalized with respect to far field strain and 90-layer thickness. The average COD is calculated using FEM in plane stress formulation. The constraint effect of adjacent layers, influence of crack spacing and the effect of out-of-plane elastic constants on COD are studied. Simple power law relating average COD to the elastic and geometrical parameters is derived based on results of parametric analysis. This relationship is successfully used to predict the axial modulus and Poisson’s ratio reduction in (±θ,904)s laminates.