The relation between morphological change, induced by copolymerized benzene rings or by kneading, and the dielectric strength of ethylene-styrene copolymer was studied. In unkneaded samples, the lamellar thickness decreases and the lattice constant along the a-axis increases with increasing styrene content. However, in kneaded samples, the lamellar thickness and the lattice constant remain constant up to a styrene content of 3.10 wt%. Spherulite is observed with a polarizing microscope in unkneaded samples, but it disappears in kneaded samples. Kneaded samples, especially the kneaded sample with a styrene content of 3.10 wt%, have higher breakdown strength than unkneaded samples. The styrene content of the sample showing the highest breakdown strength increases as the temperature decreases. At -50°C or -80°C, the sample with a styrene content of 15.40 wt% has the highest breakdown strength of all the samples. It is considered that benzene rings are moved to the interfacial region between lamellar and amorphous parts by kneading and hinder the acceleration of electrons along the lamellae. This effect of benzene rings becomes pronounced in the low-temperature region where the breakdown process is electronic.