The present invention is to provide a system for preventing unauthorized use of a mobile phone, which comprises a portable object having a radio frequency identification component installed therein and a mobile phone installed with a decoding module and an identification data, such that after the mobile phone is turned on, the radio frequency identification component and the mobile phone carry out the steps of sending out a radio signal from the decoding module; issuing an identifying data by the radio frequency identification component when being located in an effective range for receiving the radio signal; decrypting the identifying data by the decoding module after receiving the identifying data; and enabling the mobile phone to be used normally when the decrypted identifying data matches with the identification data.