A metal-oxide-semiconductor (MOS) electron tunneling cathode using n- and heavily doped p-type Si substrates were fabricated and their characteristics were measured. Two energy peaks in the energy distributions of emitted electrons from the n-type cathode with the gate oxide thickness of 8.5 nm and the p-type cathode with the gate oxide thickness of 8.9 nm were observed. This article discusses the experimental results to make a conduction mechanism clear and shows that a part of electrons tunnel through the oxide barrier from the valence band of Si substrate and are emitted into vacuum in the MOS electron tunneling cathode.