The IEEE 802.15.4 Medium Access Control (MAC) protocol is designed for low data rate, short distance and low power communication applications such as Wireless Sensor Networks (WSN). However, in the standard 802.15.4 MAC, if the remaining number of backoff periods in the current superframe are not enough to complete data transmission procedure, the sensor nodes hold the transmission until the next superframe. When two or more sensor nodes hold data transmission and restart the transmission procedure simultaneously in the next superframe, it causes a collision of data packets and waste of the channel utilization. Therefore, the MAC design is inadequate to deal with high contention environments such as densely deployed sensor networks. In this paper, we propose a data fragmentation scheme to increase channel utilization and avoid inevitable collision. Our proposed scheme outperforms the standard IEEE 802.15.4 MAC in terms of collision probability and aggregate throughput. The proposed scheme is easily adapted to the standard IEEE 802.15.4 MAC without any additional message types.