Provided is a data transmission rate control method in upward vertical handover in an overlay network environment, including: a TCP (Transmission Control Protocol) sender checking whether or not a retransmission timeout event occurs while communicating with a TCP receiver through a dual interface in a TCP scheme; the TCP sender storing a sequence number of a packet having the largest sequence number among transmitting packets, and after that, performing congestion control if the retransmission timeout event occurs; the TCP sender checking whether or not ACK signal corresponding to a sequence number which is equal to or smaller than the stored sequence number is received after the TCP sender performs the congestion control; and the TCP sender determining to execute upward vertical handover if the ACK signal is received.