Below the melting point, polyethylene forms a structure in which crystalline part and amorphous part exist together. Therefore, polyethylene is not homogeneous. It is thought that generation of a crack by the maxwell stress is one of the tree initiation causes. The crack is generated easier in a unhomogeneous stracture than in a homogeneous stracture. Above the melting point, the homogeneity may increase because all crystalline part melts into amorphous part. In order to examine the homogeneity of polyethylene and the relation to the generation of electrical tree, polyethylenewas heated up to 180°C. The relation between the superstructure and the electrical tree initiation voltage at varioustemperatures was examined.