Viscoelastic polymer blends of polybutadiene (PB)low vinyl content polyisoprene (LPI), with a lower critical solution temperature (LCST), show interesting theological behaviors in temperature ramp measurements. In this report, a systematic study has been carried out, and the underlying physics has been investigated for the storage modulus G' at various temperatures and shear frequencies as the system passes through the binodal and the spinodal phase boundary lines. We considered the nucleation mechanism, spinodal fluctuations, shear induced mixing, and theological models in the interpretation of these interesting phenomena. Shear induced mixing is varied in our system, and the frequency dependence is obvious. Competition between the kinetics of the nucleation process and the droplet growth process has a prominent effect on the storage modulus for samples of noncritical compositions, while for samples with near-critical compositions the morphological evolution is responsible for the viscoelastic changes. Time-dependent experiments provide important information about morphological evolution at different temperatures. The region where fluctuations play a dominant effect on G' can be discerned from our treatment of putting G' and {G'(omega)/[G ''(2)(omega)T](2/3) in the same reference frame. On the basis of the results from both heating and cooling processes, it seems that there also exist competition between fluctuations and interfacial gradient on the determination of the value of G'.