A cross-layer rate adaptation mechanism for wireless local area network (WLAN) can obtain the channel state by calculating the Eb/N0 ratio of ACK frame transmitted from the receiver side. The mechanism determines the transmission rate of the next frame by referring to a predefined reduced mode table. When receiving an ACK frame fails, the mechanism can automatically lower the transmitting rate of the next transmission. Therefore, the method can reduce the damage to the system when ACK frame failure happens.