Abstract: An exactly slotbased and thus accurate model for IEEE 802.15.4 protocol with sleep mechanism in real time application was proposed. The model explicitly takes into account both the sleep mechanism and CSMA/CA mechanism of IEEE 802.15.4. By dividing the superframe into backoff slots and modeling the two mechanisms with a precision of slot, it analyzes the energy consumption, throughput, and service time delay of the protocol and discusses the impact of network traffic, duty cycle, especially sleep mechanism on protocol performance. The NS2 simulations show that the model can accurately predict the performance of IEEE 802.15.4 networks. At the same time, under the same packet arrival rate, a proper duty cycle could optimize both the energy consumption and network throughput.