An aberration theory has been developed for analyzing the effects of machining errors of electrostatic ion‐optical systems comprising round lenses and octupole deflectors. The relationship has been investigated between the machining error and the induced perturbation, or the difference between the perturbed potential and the ideal one. The aberration formulas based on Sturrock’s perturbation characteristic function are shown. A computer program has been developed for analyzing electrostatic beam‐scanning systems and a submicron ion‐probe system is analyzed. The results show that lens errors may be corrected by a stigmator and that beam blurring by deflector error is negligible.