Crystalline/crystalline binary polymer blends have currently received a considerable amount of research works due to formation of many different supermolecule structures and crystalline morphologies. In polymer blends consisting of two semi-crystalline polymers with a miscibility gap, the structure and subsequently, final properties are controlled by the phase separation as well as the interplay between the crystallization of the components. In this study, blends of two semi-crystalline polymers, poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO), were prepared via solution mixing method in the whole composition range. The work focuses on liquid-liquid phase separation (LLPS) and miscibility window in PVDF/PEO blends employing different techniques. Since the phase separation may be detected by rheology even in its early stages, small-amplitude oscillatory shear measurements were carried out. Based on the Ajji and Choplin method, binodal and spinodal temperatures were determined from the abrupt changes in the slope of the storage modulus in dynamic temperature sweep experiments. An asymmetrical Lower Critical Solution Temperature (LCST) phase behavior was observed in which the system became immiscible immediately after reaching the solid-liquid phase separation temperatures in PEO-rich phase blends. The phase behavior was also evaluated using optical microscopy and the results were in good agreement with the rheologically determined phase diagram. The spinodal temperatures were also calculated based on a compressible regular solution free energy model. The comparison between the experimental results and the mathematical modeling verifies the capability of the model to predict the phase behavior qualitatively.