Blends of statistical copolymers ethylene/hexene (PEH) and ethylene/butene (PEB) show the behavior of an upper critical solution temperature (UCST). In this study, the phase diagram of PEH/PEB blends was determined by using rheological measurements, and the kinetics of phase separation was investigated from the viewpoints of polymer viscoelasticity. Based on time-temperature superposition (TTS) principle and mean field theory, rheology methods were used to determine the binodal and spinodal phase separation temperatures. Compared with optical techniques, rheology could sensitively detect phase separation in rather early stages. It was found that in either the metastable or unstable regions, storage moduli of the phase-separated blends decreased when phase separation progressed. The magnitude of G' decreasing was smaller in the metastable region than in the unstable region because the nucleation and growth process in the metastable region was dominated by diffusion and interfacial tension was much weaker, while in the unstable region, spinodal concentration fluctuations were dominant and interfacial tension was stronger. The decrease of G' in the unstable region was due to the coupling effects of reduced concentration fluctuations and decreased interfacial tension, whereas the decrease of G' in the metastable region was determined by competitive effects of the reduced interfacial area and increased deformability.