In this paper, transmission scheduling issues related to multicast traffic, which focus on multicast service efficiency, are discussed for downstream packet transmission of a time-divisionmultiplexed passive optical network. First, the features of multicast traffic-sharing and the effects on multicast efficiency are analyzed in detail. Second, metrics, which are utilized in themeasurement of multicast service efficiency, are introduced, and two kinds of fairness issues in multicast packet transmission are defined. Finally, an adaptive multicast packet transmission scheduling mechanism, called multicast-share weighted fair queuing, is proposed in order to stably guarantee fair amounts of effective throughputs to multicast sessions according to changes in the multicast service efficiencies of sessions. Analytic and simulation results confirm that the proposed mechanism guarantees the inter-session fairness in terms of effective throughput efficiently.