Abstract: The misalignment assembling error effect of lenses and deflectors and its evaluation in electron beam focusing and deflection systems are discussed. First the electrostatic and magnetic field expression in electron beam systems due to the misaligned lenses and deflectors are derived. Then the formulae for evaluating the changes of the optical properties caused by the misalignment assembling errors (i.e. the additional image shifting and additional aberrations) are given by the derived field expression and the general ray equation. A set of programs based on the fromulae given is developed, and some illustrative examples are also shown as the spot-diagrams. It can be reflected that the numerical computation of the misalignment assembling error effect plays important role in both designing and assembling electron beam focusing and deflection systems.