A data transmission method suitable for a communication system is provided. The communication system includes a plurality of first stations which uses an 802.11 protocol and a plurality of second stations which uses an 802.11e protocol and supports Quality of Service (QoS). The data transmission method includes following steps. One of the second stations is used as a Contention AssisTance (CAT) station to obtain a medium access right with a first access category (AC). The second station broadcasts a CAT packet to all the first stations and the other second stations. The other second stations stop contending for the medium access right for a first predetermined time according to the CAT packet, and the first stations transmit a packet with a QoS requirement according to the CAT packet.