The electric self-heating behavior of graphite-powder-filled high-density polyethylene is studied. Two equations are proposed to describe the electric-field dependence of the self-heating temperature and resistance dependence of the critical field. Based on Ohmic and non-Ohmic approximations and the heat-dissipation model, the self-heating equations are also derived theoretically. The equations show that self-heating is determined by the initial resistance and true positive temperature coefficient (PTC) effect under fields. Design and application principles for polymer PTC heaters are suggested on the basis of the experimental results and proposed equations.