In PONs, an optical subscriber unit (OSU) is shared by users. Since the upload/download speed depends on the available bandwidth of the OSU, the utilization ratio of OSU bandwidth must be equally distributed to offer fair upload/download speeds. A dynamic load-balancing (DLB) method to distribute OSU’s traffic load equally in λ-tunable WDM/TDM-PONs has been realized by monitoring the buffer depth of each OSU. However, it is difficult to monitor the traffic load if the OSU has an unused capacity. This paper proposes a novel DLB method for λ-tunable WDM/TDM-PONs. The proposed method makes the traffic load of OSU uniform by monitoring the traffic volume itself, even if some available capacity is present. In addition, to quickly determine the optimal OSU-ONU assignments, a determination algorithm based on the proposed method is derived from Jain’s fairness index. The proposed method is validated by simulations based on a measured traffic model.