This paper presents a new approach for determining the priority order for thermal unit commitment. In this approach, a new index CUF (commitment utilization factor) is used in conjunction with the classical economic index AFLC (average full-load cost) to determine the priority commitment order. Based on the author's application experience, the proposed approach is very effective because the resulting priority commitment orders are consistently better, whenever possible, than those determined by the AFLC index alone. This approach can be easily implemented as an add-on module to an existing priority-order-based unit commitment model. The addition can significantly improve the performance of this priority-order-based model without noticeable penalty in the computational requirements. In this paper, a midwestern utility system is used to illustrate the proposed approach.