Space-charge-formation characteristics in polyethylene, polypropylene, and their blends are discussed based on the results of DC conduction, thermal pulse current, and thermally stimulated current. Negative space charge is formed throughout the bulk of polypropylene, while positive space charge is formed in polyethylene. In polymer blends of the two homopolymers, the amount of negative space charge increases with an increase in polypropylene content, while that of positive space charge increases with an increase in polyethylene content. It appears that space charges of both polarities are formed in the polymer blends, presumably with a tendency for the charges to be trapped in their respective preferential polymers. By blending the two homopolymers, DC conduction current becomes larger than that of homopolymers. The dielectric strength at low temperature which is electronic in nature decreases by blending. The dielectric strength at room temperature which is governed by a thermal process increases by blending the two homopolymers, because the spherulite boundaries of polypropylene are reinforced to improve the mechanical properties.