In this article we present some techniques to determine the types of automorphisms of extremal doubly even binary self-dual codes, also called ex- tremal type II codes, with parameters (24,12,8), (48,24,12) and (120,60,24). We aim to obtain information about the automorphism group considering the exclusion of some prime numbers from its order. Keywords: Binary codes, self-dual codes, doubly even codes, extremal codes and automorphisms of codes. MSC2010: 11T71, 20B25, 94B60.