In Mandarin, there is a special acoustic feature—tonal pitch range, which is relative to stress. In this paper, we present a novel concept—tonal range ratio (TRR), which is based on tonal pitch range, and make a study on the correlation between TRR and stress in Mandarin. And we developed a system to automatically detect stresses in words and sentences based on TRR in Mandarin. We obtained high success rate (92.67 % in words and 82.0 % in sentences). The results show that TRR has strong correlation with stress and is powerful in detecting stresses in Mandarin. 1