Constitutive laws are fundamental to the studies of the mechanically dominated clinical interventions involving soft biological tissues which show a highly anisotropic hyperelastic mechanical properties. The purpose of this paper was to develop an improved constitutive law based on the Holzapfel-Gasser-Ogden's model: to replace the isotropic part with Gent constitutive law so as to model the noncollagenous matrix of the media due to its generality and capability to reproduce the Neo-Hookean model. This model is implemented into an in-house finite element program. A uniaxial tension test is considered to study the influence of material parameter J(m) in Gent model and β which represents the angle between the collagen fibers and the circumferential direction. A simulation of an adventitial strip specimen under tension is performed to show the applicability of this constitutive law.