Mixtures of ionic liquids and polymers are of significant interest for a variety of possible application e.g., polymer electrolytes, nanoencapsulations and actuators. Understanding the relationship between structures and electrochemical properties is of crucial importance for realizing these applications. In this work, the effect of phase separation on crystallization is investigated for poly(ethylene oxide)/ionic liquid mixtures that have a lower critical solution temperature. It was demonstrated that the crystallization kinetic is highly enhanced by phase separation as the samples are quenched from two-phase region and isothermally crystallizes at a given temperature. Furthermore, the facilitating effect on the crystallization of PEO will be more obvious with a longer annealing time above the phase separation temperature, over a certain time. On the contrary, the crystallinity of these samples is not increased, and even has a slight decrease compared with the sample quenched from a single-phase region. This phenomenon can be ascribed to that the cage on the PEO chains, due to the hydrogen bonds, inhibits the migration of these chains, and thus retards the crystallization of PEO, as the mixture quenched from a single-phase region. On the other hand, annealing above the phase separation temperature causes an increase of chain mobility and a decrease of free nucleation energy, thus a highly enhanced crystallization kinetics of PEO by phase separation occurs. It could be imagined that this method may be helpful to reduce the time to prepare polymer electrolytes, and meanwhile do not lower and even slightly increase the ionic conductivity of the electrolytes.