In this paper, we analyze the operation of ZigBee devices under the presence of WLAN 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 utilize the fact that the transmission power of WLAN devices is set hundred to thousand times higher than that of ZigBee devices. Thus, we assume WLAN devices interfere with ZigBee devices, but the ZigBee devices do not. The throughput of a ZigBee network is derived for varying the input load of WLAN interference. The result shows that analytical results obtained from the mathematical model agree well with simulation results.