This paper proposes a power management technique for video playback in mobile multimedia devices, which combines low-power video decoding and QoS-guaranteed algorithms: ILI (Interval-based Linear Interpolation) and QLB (QoS-guaranteed and Low-power Buffering). First, the proposed ILI algorithm precisely estimates the decoding time of video frames through interval-based linear interpolation. Second, the QLB algorithm has two modes of operation, and dynamically switches between the modes based on feedback of decoding statistics of recent frames. The combinations ILI and QLB algorithms allow the CPU voltage and clock frequency to be adjusted to the optimal setting. Our extensive experiments show that the proposed approach can achieve about 4.8%-13.7% more power savings compared with existing methods.