The electron voltaic effect of semiconductor was first studied by Becker and Kruppke (see ref. 1) and later by Ehrenberg, Lang and West (see ref. 2 & 3). They found that the e.m.f. developed on the photo element under the bombardment of electron beam increases at first rapidly with accelerating voltage, reaches a maximum and then decreases. The accelerating voltage corresponding to the maximum e.m.f. developed on the cell was found to be various from cell to cell. In this paper a detailed theory of electron voltaic effect was developed under the assumption that the primary current of the electron beam is so small that a linear appoximation may be used. The Thomson-Whiddington law is used for getting the energy dissipation of the electron beam in semiconductor. The results of the present theory show that the e.m.f. developed on the cell appears a maximum, when the depth for which the incident electrons penetrate into the semiconductor is of the same order of the diffusion length of the excited electrons, rather than the thickness of the barrier layer. The numerical calculation is carried out for some typical examples and the results are discussed and compared with the experimental data.