An apparatus and method for Transmission Time Interval (TTI) reconfiguration in a mobile communication system are provided. The apparatus includes an ACK/NACK determiner, an ACK/NACK controller, and a TTI controller. The ACK/NACK determiner determines if a response signal to be transmitted is ACKnowledgment (ACK) or Non ACKnowledgment (NACK) depending on an error or non-error of data received from a User Equipment (UE). The ACK/NACK controller counts the ACK and NACK determined in the ACK/NACK determiner. The TTI controller identifies if number of the latest ACKs counted in the ACK/NACK controller is continuously generated by preset number of times and, if it is continuously generated by the present number of times, reconfigures a new TTI.