IEEE 802.15.4 which is used as the Low Rate(LR)-Wireless Personal Area Networks (WPAN) standard supports Guaranteed Time Slot(GTS) allocation scheme to ensure the Quality of Service(QoS) in the Contention-Free Period(CFP). However, it must be competitive in the same way as other devices via Carrier Sense Multiple Access(CSMA)-Collision Avoidance(CA) in order to request the assignment of the GTS and possible assignment of up to seven GTS. In addition, multi hop devices from the PAN coordinator are not easy to be guaranteed QoS through the GTS. In this paper, we propose not only the scheduling algorithm of GTS allocation but also MAC frame format for resolving transmission problems in multi-hop wireless sensor networks. With proposed schemes we can offer the more efficient use of GTS than the previous one and guarantee the QoS without the waste of symbols.