Abstract: The invention relates to a method and an equipment of preventing the misuse of a copied subscriber identity in a mobile communication system. The use of a subscriber identity is restricted to predetermined terminals, each having an associated equipment identity in such a way that: a number of allowable equipment identities is defined for at least one subscriber identity, and a data base linking said subscriber identities and the equipment identities allowable to them is created; the terminal sends the terminal user subscriber identity and its dedicated equipment identity to the rest of the system; the rest of the system compares whether the equipment identity sent is allowable to the subscriber identity sent; as a response to the equipment identity sent being allowable to the subscriber identity sent, the use of the terminal is permitted, and otherwise the use is denied.