The geometrical accuracy in grinding is affected by various factors, and the geometrical accuracy generation process should first be made clear by ana-lyzing the grinding process under the existence of each factor in order to improve grinding geometrical accuracy. From the above mentioned point of view, the grinding process with unbalanced wheel has experimentally been analyzed to point it out that the runout is. generated on the workpiece surface by the unbalance of wheel, and that the runout generation process is significantly affected by the revolution ratio of wheel to workpiece. The effects of grinding conditions upon the amplitude of runout on the workpiece surface have then been analyzed to make it clear that the runout becomes smaller with smaller unbalance of wheel, lower plunge speed and finer dressing.