In this paper, we propose a QoS guaranteed and energy-efficient transmission scheme for Wireless Personal Area Networks (WPANs), which operate in conjunction with contention-based access protocols such as CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). Energy consumption is one of the most important issues in WPAN systems, because WPAN devices are often required to operate under limited battery capacity. Furthermore, if the WPAN adopts a contention-based medium access protocol, the energy consumption problem becomes even more critical due to the collisions caused by independent channel access attempts. Therefore, in this paper, we propose an algorithm that selects the optimum fragment size, modulation level, and transmission power, in order to minimize the energy consumption and guarantee the QoS (Quality of Service) requirements, simultaneously. From the simulation results, it is shown that the proposed algorithm can achieve the minimum energy consumption, satisfying throughput and delay requirements.