IEEE 802.15.4 is a standard of Low-Rate Wireless Personal Area Network (LR-WPAN), which provides the communications in short distance, low data rate, low power consumption. In IEEE 802.15.4 a guaranteed time slot (GTS) structure is defined to support high quality of service (QoS) to time-critical traffic. However, the transmission bandwidth of GTS may be wasted because of the inefficient allocation. Moreover, the coordinator in the PAN allocates the GTSs by first-come-first-serve (FCFS) scheme, which decreases the system performance, such as delay time and throughput. In this paper, we proposed a Multi-Factor Dynamic GTS Allocation Scheme (MFDGAS) to improve the utilization of GTS bandwidth. The MFDGAS scheme determines allocation of GTS by taking the data size, delay time and the utilization of GTS time slot into consideration. The numerical results show that the proposed MFDGAS improves the performance of system throughput.