A micromechanically-based constitutive model for high density polyethylene (HDPE) in small deformations is presented. The microstructure of HDPE consists of closely packed crystalline lamellae separated by layers of amorphous polymer. Here a semi-crystalline polymer is modeled as an aggregate of randomly oriented composite inclusions, each consisting of a stack of parallel lamellae with their adjacent amorphous layers. For the amorphous phase, the viscoelastic constitutive behavior is modeled, assuming a polydomain liquid-crystal-like structure and micromechanical parameters such as the elastic constant of distortion and the persistent length of polymer molecules are used. The viscoplastic behavior at yield is incorporated through the constitutive modeling of the crystalline lamellae. Constitutive equations for the composite inclusions are proposed and different homogenization schemes for the overall properties discussed. The intermediate phase linking the lamellae and the amorphous layers is assumed to form a surface layer around each lamella and its role in the yield behavior of HDPE is discussed.