Wireless sensor networks (WSNs) are applied to more and more fields such as healthcare, building automation, environment monitoring, industrial control, etc. For these applications, a high-confidence wireless communication protocol is crucially important, which should be self-adapting, real-time, dependable, and compatible with other protocol standards. IEEE 802.15.4 is a popular choice for MAC/PHY protocols used in WSNs. Therefore, it is imperative to evaluate its performance. In this paper, we will analyze the performance of the IEEE 802.15.4 standard operating in non-beaconed mode. Extensive simulations have been done to examine how the network performance in terms of metrics such as latency and packet loss rate will be impacted by some key parameters. The results are presented and analyzed. The analysis of simulation results can provide support for parameter configuration and optimization in WSN design.