The microstructures of polyethylenes (PEs) were investigated by differential scanning calorimetry (DSC). Homogeneous and inhomogeneous blends of three types of PEs, that is, low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), were used for measurement. The thermal behavior of samples suggests that homogeneous blends of HDPE and LDPE or LLDPE have a metastable microstructure which is caused by the co-crystallization of two different types of PEs. The single endothermic peak, due to the fusion of the homogeneous blend, is observed in the first heating and is not completely split into the peaks of respective components PEs by the second heating. From the result, it is thought that the molecular chains of PEs are entangled during the co-crystallization and they are not completely loosened by the first melting. The relation between the microstructures and the thermal behavior revealed by DSC has been discussed.