Blends of two semi-crystalline polymers, poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO), were prepared via solution mixing method over the whole composition range. The research focuses on the liquid-liquid phase separation (LLPS) and miscibility window in the PVDF/PEO blends by employing several techniques. Small-amplitude oscillatory shear measurements were carried out to detect LLPS and determine spinodal and binodal decomposition temperatures in its early stages. An asymmetrical lower critical solution temperature (LCST) phase diagram was observed from dynamic temperature sweep experiments in which the system became immiscible after the solid-liquid phase transition (SLPS) temperatures in the PEO-rich blends. In fact, a one-way partial miscibility of PEO in the PVDF-rich phase blends was observed. The rheologically determined phase diagram was also verified by means of optical microscopy and cloud point technique. The results revealed that the PVDF/PEO blends undergo a viscoelastic phase separation (VPS). This was attributed to the large viscoelastic asymmetry of the components in the blend. A compressible regular solution free energy model was employed to determine the spinodal temperatures theoretically. The comparison between the experimental results and the mathematical modeling indicated that the model is able to predict the phase behavior qualitatively.