We propose a real-time message scheduling algorithm which is applied to schedule periodic realtime messages in IEEE 802.15.4 for LR-WPAN(Low Rate Wireless Personal Area Network). The standard allows GTSs (Guaranteed Time Slots) in the optional use of a superframe structure in a beacon-enabled network to be used to exchange real-time messages. To utilize these features of the standard efficiently, a proper message scheduling algorithm is needed. The proposed off-line message scheduling algorithm which is based on a distance constrained scheduler generates the standard specific parameters including BO, SO, and GTS information to schedule the given message set. The algorithm is evaluated by simulation and the guaranteed time service using the schedule is implemented and evaluated on CC2420DB which is a prototyping platform including an IEEE 802.15.4 compliant transceiver of Chipcon AS.