Phase separation kinetics of polyethylene copolymer blends polyethylene-co-hexene (PEH)/polyethylene-co-butene (PEB) at a phase separation temperature of 130 degrees C have been investigated through the combination of rheological measurements and optical microscope observation. When the blends are located in the unstable region, i.e., PEH/PEB 40/60 blend (H40), 50/50 blend (H50), and 60/40 blend (H60), due to the coeffect of the fast decay of concentration fluctuations and the reduced interfacial area, the stroage modulus, G', behaves dramatically, decreasing at the early or intermediate stages; while when the blends are located in the metastable region, i.e., PEH/PEB 70/30 blend (H70), G' decreases slightly and slowly during the whole time sweep process. During the cyclic frequency sweeps, G' evolutions of H50 and H70 show similar trends. Obviously different from the strong phase segregation systems, the increase of G' with time in the metastable region has not been observed, possibly due to the entanglement effects and weak interaction between the components of polyethylene blends. The interfacial tension-driven or diffusion-limited morphological evolutions of H50 and H70 during phase separation give direct interpretations to the viscoelastic difference between the two blends, which is dominated by different phase separation kinetics. The relatively low interfacial tensions at the late stage of phase separation for H50 (0.5-0.38 mN/m varying with time) and H70 (1.2 mN/m) can be estimated by using the Gramespacher-Meissner model.