Quality of Service (QoS) support has become a key factor in designing Media Access Control (MAC) protocols. This paper introduces a novel scheduling scheme which supports priority based QoS in Wavelength Division Multiplexing (WDM) star networks. The proposed Interval-based Prioritized Orderly Scheduling Strategy (IPOSS) employs a collision-free scheduling approach and handles variable-length data packets. In practice, it is designed to handle real-time traffic, on the basis that each node may generate high- and low-priority packets with high-priority packets being scheduled prior to low-priority ones. Moreover, the proposed scheme differentiates the packets' schedule order, by prioritizing the long-length over the short-length packets. The performance of IPOSS is evaluated under Bernoulli traffic and simulation results indicate that the novel scheme achieves a significantly high throughput-delay performance for real-time traffic, without sacrificing the performance for non-real-time traffic.