The viscosity development associated with the microstructure of various crystallizable polymer blends has been investigated. All binary blends, consisting of crystallizable polyester and noncrystallizable polyether, exhibit liquid/liquid phase-separated structures with polyester-rich and polyester-poor phases. The dispersed domains-continuous matrix morphology was observed in blends with 10-40 wt% polyester, while the 50 wt% polyester blend exhibits bicontinuous morphology. Differential scanning calorimetry and optical microscopy were employed to characterize crystallization features of each phase. "Curve-leaf"-shaped crystallites were formed in the polyester-poor phase in all blends studied. The viscoelastic physical gelation of the phase-separated blends with low polyester content of 10-40 wt% was found to be dominated by crystallization in the polyester-poor phase. The unusually high viscosity of the polyester-poor phase is due to the percolated morphology of "curved leaf"-shaped crystallites.