In this paper, we analyze the operation of ZigBee devices under the presence of Bluetooth interference and formulate a mathematical model for evaluating the performance of ZigBee devices. Our proposed mathematical model for ZigBee devices is based on a Markov chain concept. We here consider Bluetooth devices which adopt class 1 or class 2 Bluetooth chips as an interference source and the transmission power of Bluetooth devices is set several to hundred times higher than that of ZigBee devices. Thus, we assume Bluetooth devices interfere with ZigBee devices, but the ZigBee devices do not. The throughput and energy consumption of ZigBee devices in a ZigBee network are derived for varying the number of actively interfering Bluetooth devices. The result shows that analytical results obtained from the mathematical model agree well with simulation results.