A system for preventing IP (Internet Protocol) allocation to a cloned mobile communication terminal is provided, including: a mobile communication terminal generating a hash value obtained by hashing a user key inputted by a user and an ESN (Electronic Serial Number) with a hash function, and transmitting the hash value to an HA (Home Agent) of a mobile communication system; and an HA registering the hash value transmitted from the mobile communication terminal, comparing a hash value included in an IP registration request message (RRQ) with the registered hash value at the IP registration request of the mobile communication terminal, and performing an IP registration process if the compared values match.