A congestion control method and system in a communication system that transmits and receives a data segment using Transmission Control Protocol (TCP) and performs a Fast Retransmit mode and a Fast Recovery mode upon occurrence of a lost segment. A transmission terminal retransmits the lost segment to a reception terminal and then calculates the expected number of duplicate acknowledgements (ACKs) expected to be received from the reception terminal. The transmission terminal determines whether an ACK recently received from the reception terminal is identical to a new ACK or a duplicate ACK. The transmission terminal accumulates the received duplicate ACK, and compares the number of the accumulated duplicate ACKs with the expected number of the duplicate ACKs. The transmission terminal retransmits the lost segment to the reception terminal if the number of the accumulated duplicate ACKs is larger than the expected number of the duplicate ACKs.