In this paper, we propose a deferred-workload-based inter-task DVS (dynamic voltage scaling) algorithm (dwDVS), which has two features, for portable multimedia devices. The first is that we reserve a time interval for each task to execute and its workload can be completed in this time interval even in the worst-case condition, which means that the actual workload (execution time) of each task is equal to its worst-case execution time. In this way, we can estimate the slack time from lower priority tasks more aggressively. The second is that we defer these reserved time intervals, which means that a reserved time interval will be shifted to the deadline of its corresponding task as close as possible. Thus, the operating frequency can be reduced even without slack time. Simulation results show that the proposed dwDVS reduces the energy consumption by 40-70%, 10-20%, and 3-10% compared with the static voltage scaling (Static), laEDF, and DRA algorithms, respectively, and approaches theoretical low bound (Bound) by a margin of at most 12%.