The objective of this study is to assess the blood oxygenation oscillations in the tissue over the sacrum using spectral analysis of near-infrared spectroscopy (NIRS) signals based on wavelet transform. A total of twenty subjects were recruited for this study, of which ten were persons with spinal cord injury (SCI) (3 women, 7 men, age=34.5±5.2 y) and the other ten were healthy subjects (3 women, 7 men, age=36.5±6.2 y). External pressure of 26.6kPa (200 mmHg) was applied to the sacrum via a specifically designed indentor. The loading duration was 3 minutes. The subjects were examined lying face-down. Tissue oxygenation signal was monitored for 20 minutes prior to and after the loading period from the tissue over the sacrum area using NIRS. The results showed that the amplitude of the [HbO2] and [Hb] component in intervals I, II and III for persons with SCI was significantly lower during the resting conditions than that for normal subjects (p>0.05). During the post loading period, the response of [HbO2] and [Hb] oscillatory activities in intervals II and III were significantly lower in the tissue over the sacrum for persons with SCI than that for normal subjects (p>0.05). The results indicated that the low oscillatory activities might be related to early tissue injury in persons with SCI.