A method for detecting an erroneous sequence number in a status report in a wireless communications system includes receiving a status report sent from a receiver of the wireless communications system, detecting whether a negatively acknowledged sequence number is within a range that is larger than or equal to the sequence number following the sequence number of the last in-sequence acknowledged packet and smaller than the sequence number of the next packet to be transmitted for the first time, and detecting that the status report includes an erroneous sequence number when a negatively acknowledged sequence number is not within the abovementioned range.