In this paper we present an analysis of a MAC (Medium Access Control) protocol for wireless sensor networks. The purpose of this protocol is to manage wireless media access by constructing a Time Division Media Access (TDMA) schedule. APMC (Approximate Probabilistic Model Checker) is a tool that uses approximation-based verification techniques in order to analyse the behavior of complex probabilistic systems. Using APMC, we approximately computed the probabilities of several properties of the MAC protocol being studied, thus giving some insights about it performance. Key words: Wireless sensor networks, approximate verification