The highprecision radial grating has been widely used as orientation standard in highprecision angular measurement instrument. This paper discusses how to increase the precision of radial grating and introduces a method which corrects the dividing error of radial grating automatically. Through analizing the dividing errors of radial grating, the variation of main error sources can be seen and its mathematical expression can be derived. Microcomputer is utilized to control a special transmission device which makes radial grating rotate an additional angle and counteract the dividing errors of the radial grating.