An apparatus, system, and method for discovering a path MTU in a data communication network, wherein a path MTU option header is included in a signal message or first data packet and thus a MTU suitable for a routing path can be determined. A network apparatus for discovering a path MTU maximum transmission unit (MTU) in a data communication network, including a MTU-comparing unit MTU-comparer detecting a path MTU recorded on a predetermined packet transferred over a routing path between a source node and a destination node and comparing the detected path MTU with an internal MTU stored in the MTU-comparing unit MTU-comparer; and an event-processing unit event-processor updating the path MTU recorded on the predetermined packet to contain the internal MTU if the detected path MTU is larger than the internal MTU.