An International Mobile Equipment Identification (IMEI) is coded and then stored in a user equipment (UE), wherein an authorized user is allowed to change the existing IMEI. When an IMEI is inputted, the UE generates a security code and stores the IMEI and the generated security code in a memory. Thereafter, if a second security code and IMEI is inputted, the UE determines whether the inputted security code is identical with the pre-stored security code. If the two security codes are identical with each other, the UE generates a third security code and stores the IMEI and the generated third security code. Thus, unauthorized IMEI storage is prevented. Accordingly, the IMEI coding method of the mobile UE can prevent unauthorized use of the UE while allowing the IMEI to be re-coded without hardware alteration.