Electrothermal processes were studied in pyroresistive composites based on high-density polyethylene (HDPE) containing 8 vol.% carbon black (CB), 8 vol.% carbon fibers (CF), and their mixture 4 vol.% CB + 4 vol.% CF. It is shown that the kinetic heating curves of composites are well described by an exponential dependence with a certain heating rate constantkfor each type of composite. After a short heating time, the equilibrium temperatureTeis reached in the sample. When the applied voltage exceeds a certain value, theTevalue decreases due to the presence of the positive temperature coefficient of resistance (PTC) effect. Due to the PTC effect, the composites exhibit a self-regulating effect relative to theTe. Relations between the applied voltage, electric power, and equilibrium temperature are found, theTevalue depends on the applied voltage according to the quadratic law whereas there is a linear relationship between theTeand electric power. A possible application of such pyroresistive composites is resistance welding of plastics using a heating element (HE) made of a pyroresistive material. The use of HDPE-CB composite to create HE for resistance welding is demonstrated and the welded joint of HDPE parts obtained using HE is shown.