A method for sending an ACK from the receiver to the sender is disclosed. The receiver starts an ACK timer with the ACK timeout value being smaller than the RTX timeout value. The ACK timer is stopped or cancelled when an ACK is sent from the receiver to the sender. When the number of the delayed ACK for I-frames is close to the receive widow size of the receiver or the ACK timer is timeout, the receiver sends an ACK to the sender actively. When the receiver receives a Poll frame from the sender due to RTX retransmission timeout, the receiver sends an ACK to the sender passively. The receiver adjusts the ACK timeout value used next time based on the following: timeout value of a current ACK timer, RTX timeout value of the sender, the receive widow size of the receiver and the number of I-frames for which ACKs have not been sent, in order to reduce the S-frames and stops of data transmission, thus improving the data transmission speed and the bandwidth utilization.