An apparatus to set a maximum transmission unit (MTU) acquires information about a wireless communication network, sets a basic MTU value based on the kind of wireless communication network as an MTU value, and checks the validity of the set MTU value. The validity of the set MTU value is checked by dividing packets by the set MTU value, transmitting the packets to a communication counterpart, and comparing a response message's error rate to a reference value. The apparatus increases the set MTU value if the set MTU value is valid, decreases the set MTU value if the set MTU value is not valid, and detects an optimal MTU value. The optimal MTU value may refer to a longest MTU value among checked MTU values that has an error rate less than the reference value according to the validity-checking process.