<p>Six Sigma indices are widely used in the manufacturing industry for quality assessment, analysis, and improvement. The Six Sigma quality index and the Six Sigma Taguchi index can simultaneously reflect quality levels as well as process capability. However, although process yield is a key factor of process quality, neither the Six Sigma quality nor Taguchi indices have a one-to-one mathematical relationship with process yield. Therefore, process yield must be assessed separately. This paper thus proposes a Six Sigma process yield index, which not only reflects the process quality level and degree of process capability but also has a one-to-one mathematical relationship with process yield. In addition to discussing characteristics of the proposed index, we make statistical inferences and provide an evaluation model for industrial use.</p>