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The IEEE 802.15.4/ZigBee is developed to meet the needs in wireless personal area networks by adopting CSMA/CA algorithm as the retransfer mechanism after the collision occurred. But existed evaluation for 802.15.4 mainly bases on simulation and the only few existed mathematical models are either developed immaturely or based on inaccurate assumption, thus failing to meet the rapid evolution and increasing complexity for low-rate wireless applications such as wireless sensor networks (WSN). Therefore, to evaluate the performance of 802.15.4 protocol and better utilize its strength for low-data-rate wireless application, this paper presents an accurate two- dimensional discrete Markov chain model for evaluating the 802.15.4 CSMA/CA mechanism, focusing on the analysis of relationship between throughput and related parameters. By building a Markov chain for the backoff counter, computing the stable distribution probability and success transmission probability in channel collision, this paper finally gives the throughput and energy consumption formula. In the experimental part, we use ns2 simulator to evaluate the performance of 802.15.4 at the 2.4 GHz scale under different circumstances and the final performance well validates our theoretical conclusion. Considering the former work mainly bases on simulation and existing models lack accuracy, our theory and simulation combined analytical model is both novel and valuable.