This paper is about positioning error of photomask by resist charge up effect in the EB lithography. We postulated that charges created by incident electron beam in the photomask form electric dipoles perpendicular to its surface by the electron image effect from metal film. The formed electric dipole distributions depending on writing patterns deflect the orbit of the electron beam . We have simulated the deflection of the electron beam by the dipole produced at the surface and obtained the dipole distribution that led to the experimentally measured position error for a test writing pattern. Our model will be useful to predict the positioning error in EB lithography.