THE SCOPE OF THIS ARTICLE CONCERNS TO GEOMETRICAL DESIGN MODIFICATION OF DISC INSULATOR 120KN, MANUFACTURED BY IRAN INSULATOR COMPANY. IN THIS ARTICLE IN THE INTERIM OF DISCUSSION ON THE PRINCIPLES OF ELECTRICAL SPECIFICATION OF DISC INSULATORS, ON THE SURFACE OF THE PREVIOUS DESIGN AND NEW DESIGN A FEASIBILITY STUDY HAS BEEN DONE AND THEN FOLLOWED ANALYST AND DISCUSSION OF ITS ELECTRICAL FIELD DISTRIBUTION.IN CONTRAST TO PREVIOUS GEOMETRY DESIGN OF STANDARD DISC INSULATORS, ACCORDING TO STANDARD WITH HAVING A LIMITATION IN CREEPAGE DISTANCE AND DISC DIAMETER, ALL THE AFORESAID POINTS IN NEW MODIFIED DESIGN IS CONSIDERED. STUDY ON ELECTRICAL FIELD DISTRIBUTION SURVEY HAS BEEN PERFORMED BY ANSYS F-ELEMENT, SOFTWARE AND USED BOUNDARY CONDITION IS CALCULATED ACCORDING TO ITS USED CONDITION IN OVERHEAD LINES. THE MODIFIDED DISK INSULATOR HAS GOT TYPE TEST CERTIFICATE FROM IEN LABORATORY IN POLAND ON FEB,2004.