The objective of this work is to propose an analytical model, which is based on a fabric flexible composite theory, to characterize soft tissues (such as heart muscle, skin, lung, blood vessel, etc.). The bi-axial mechanical behaviors of the soft tissues have been studied by many researchers. A common approach is to propose a set of constitutive equations, then use data regression method to obtain input values from both simple and bi-axial test results. This usually gives a good description of the stress-strain behavior for the particular material but gives no physical meaning of these input values obtained from the mathematical curve fittings. Pao et al. (1980) suggested that the mechanical properties of the fiber bundles must be predicted first to compute the stresses. In this proposed model, a soft tissue is considered as an orthogonal composite element containing wavy fibers in a soft incompressible matrix (Luo and Chou, 1990).