The morphology and rheological behavior of cyclic olefin copolymer (COC) blends with two molecular weights of polyhexene-1 (PH-1, PH-1-UH [ultra-high]) were investigated at a wide range of compositions. Morphology of the blends at low concentrations of polyhexene-1 s showed a droplet-matrix structure and changed to a co-continuous morphology at intermediate concentrations. The rheological Cole-Cole plots and viscosity versus composition confirmed immiscibility of the blends. The interfacial interaction of blends phases was investigated and Complex viscosity and storage modulus versus frequency were measured and the results were consistent with high interfacial strength between the COC and high concentrations of PH-1-UH which increases the melt strength of the blends. Relaxation time spectra and Tan delta versus frequency curves were analyzed and these results were consistent with a high degree of entanglements between the COC and PH-1-UH chains in the PH-1-UH-rich compounds which in turn increased the elasticity. Damping factor measurements and calculation of interfacial tension using emulsion models showed that in the COC-rich blends, the interfacial interaction COC/PH-1 blends is higher than that of the COC/PH-1-UH blends and thus the elasticity and particles form relaxation time of the PH-1 blends are higher compared to PH-1-UH blends.