Disclosed is a method of preventing a replay attack during a handoff in a communication system using a Mobile IPv6 protocol. A mobile node creates a CoA (Care of Address) by handoff and sends the CoA to a correspondent node, thereby creating a binding entry. Upon receipt of a binding update message including a HoA and a CoA from the mobile node or an attacker, the correspondent node searches a binding cache for a binding entry having the same HoA (Home Address) and CoA as included in the binding update message. If it is determined that the binding entry has the same respective HoA and CoA as contained in the binding update message, the correspondent node checks a Used field of the binding entry. If it is determined that the Used field of the binding entry is equal to a predetermined value, the correspondent node sends a reauthentication command message to the mobile node.