To construct constraint-free human-computer interaction (HCI) environments, the wireless communication is one of the most essential research issues. Among the numerous wireless communication technologies, IEEE 802.15.4 low-rate wireless personal area networks (LR-WPANs) is the most suitable for ubiquitous and pervasive HCI environments, such as home networking applications, monitoring systems, and so on. In LR-WPANs, the power-efficiency is one of the most attractive issues. Therefore, this paper proposes a power saving medium access scheme for star topology beacon-enabled LR-WPANs. To achieve power efficiency, we turn off communication modules when the time slots are not used for data communications. On the other hand, we turn on the modules. The proposed scheme predict the time for data communications in a given superframe structure. Our simulation result shows that our algorithm achieves desired properties.