The design equations for pipelines subjected to both internal pressure and longitudinal loading are based on the isotropic hardening plasticity model. However, high strength steel (HSS) pipelines exhibit plastic anisotropy, which cannot be incorporated in the traditional isotropic hardening plasticity model. The stress strain behaviors of HSS in the longitudinal and the circumferential directions are different. Thus, it would not be desirable to adopt the same design equations based on the isotropic hardening plasticity model for HSS pipelines. The design equations of HSS steel pipelines have to be developed by solving numerical models incorporating a suitable material plasticity constitutive model for the HSS that can deal with the exhibited plastic anisotropy. In this paper, various plasticity models are studied and an appropriate plasticity model is adopted and calibrated to model the plastic anisotropy exhibited by the HSS.