The elongational rheological behaviors of glass fiber (GF) filled poly-(ethylene-co-alpha-octene) POE and long chain branching POE (LCB POE) composites were investigated, with the fiber concentration varying between 0 and 50 wt.%. Three initial fiber orientations, namely planar random, parallel to the stretching flow direction and the perpendicular to it, were adopted to study the mechanism of strain hardening. The fiber orientation distributions of the samples at various Hencky strains were also determined by the optical microscopy and image analysis techniques. It is found that fibers orientate to the stretching direction during elongation and the state with fibers parallel to the flow direction has the highest elongation viscosity, which accounts for the strain hardening in fiber-filled composites. In LCB POE/GF composites, the strain hardening due to fiber orientation starts earlier than that due to chains stretching, but the latter will dominate at larger strain. Two methods were defined to evaluate the strength of strain hardening, which was found to depend on the fiber concentration, initial fiber orientation, and viscoelasticity of polymer matrix. The results were compared with two constitutive models, the Lipscomb et al.'s model and Ferec et al.'s model, which could semi-quantitatively describe the effect of fiber concentration and initial fiber orientation but failed to predict the effect of polymer viscoelasticity.