A static magnetic field is applied during the hot press molding of high-density polyethylene (HDPE)/carbon black (CB)/ferrous powder composites and HDPE/CB/carbon nanotube (CNT) composites. The effect of magnetic field treatment on the temperature-sensing property of polyethylene-based positive temperature coefficient (PTC) composites is investigated. Experimental results indicate that the magnetic field treatment can lead to the alignment of ferrous powders and CNT in HDPE along the direction of the applied external magnetic field, thus guaranteeing the composites have low room-temperature resistivity and high PTC intensity as the concentration of CB is cut down. This also can significantly improve mechanical property and processability of the composites. When the mass concentrations of ferrous powders and CNT are equal, HDPE/CB/ferrous powder composites present better PTC property than HDPE/CB/CNT composites after the magnetic field treatment.