Abstract. In a wireless sensor network, a few sensor nodes can be physically captured by a malicious adversary and then, by using the whole information they hold, skillfully modify some legitimate packets passing through the captured nodes. Such packet modification attacks (PMAs) could make real events on the field or control messages from the base stations (BSs) considered as false ones, so that some critical events, such as the enemy's advances, may not be reported to the BSs, or BSs may lose control of numerous nodes. In this paper, the author proposes a method of detecting PMAs in WSNs, in which each PMA can be detected and notified to the BSs through a time division multiple access based collaboration among the detecting nodes. While the proposed method does not require tamper-resistant hardware, the method guarantees that a PMA can be detected unless half of the detecting nodes are collude with the PMA. The performance of the method is reviewed with simulation results.