Recently, TDMA-based MAC protocols have been discussed in WSNs for their energy-efficient performance. In the use of TDMA-based MAC scheme, radios of sensors can be turned off during idle times in order to conserve energy. Clustering is an effective method for achieving high levels of energy efficiency and scalability, and is often combined with TDMA-based schemes to reduce the cost of idle listening. But, conventional TDMA protocols focus on intra-cluster communication MAC control. They don't consider collisions occurring among inter-clusters in developing architecture for WSNs that is extensible to hundreds of thousands of nodes. This paper proposes a TDMA based MAC protocol for avoiding inter-cluster collisions which avoids collisions occurring among clusters.