Abstract: Number of MAC protocols have been proposed for Underwater Acoustic Networks (UANs). However, unique characteristics of acoustic communication such as very slow data rate inconsistent long propagation delay and long physical header degrade the performance of the most existing works. In order to improve network performance, COD-TS has been proposed. COD-TS utilizes clustering algorithm to organize nodes into clusters. To avoid inter-cluster collisions, cluster heads exchange schedule information between them based on CDMA. COD-TS has two problems. Near-far effect has not been considered for schedule information packets. It exchanges schedule information at high power, causing collisions to member nodes in the neighboring cluster. Therefore, it causes degradation of network performance. We introduce a cluster based underwater TDMA protocol (CU-TDMA) which is collision free and energy efficient. In the CU-TDMA, first an UAN is partitioned into layers based on water pressure. Then, nodes in each layer are organized into clusters. To address the inter-layer inter-cluster and intra-cluster collisions, we present a TDMA algorithm. We compare the performance of the proposed protocol with the COD-TS in simulations. The results show that the proposed protocol outperforms the COD-TS protocol in terms of throughput and energy efficiency.