An electrical insulator and its related battery are provided. The electrical insulator includes a separation region and a supporting region. The supporting region covers the edge of at least one of opposite surfaces of the separation region and at least a part of a lateral surface of the separation region. The battery includes the electrical insulator. The electrical insulator is disposed between cathode and anode active material layers of the battery and contacts the active material layers directly. The cathode and the anode active material layers are completely electrically isolated because the electrical insulator totally covers at least one active material layer. Hence, the inner short of the battery is prevented.