The objective of the present study is to assess the effect of pressure on blood oxygenation in the sacrum tissue (high-risk area for pressure ulcer) based on near infrared spectroscopy (NIRS) signals. NIRS was used to detect the change in the value of blood oxygenation. Thirty subjects were recruited, of which ten were elders (average age, 73.4 y ), ten were persons with spinal cord injury (average age, 32 y) and ten were healthy persons (average age, 25 y). In resting conditions, the blood oxygenation in sacrum tissue of the 30 subjects was monitored for 20 minutes prior to and after the three-minute loading period. The results show that the first three oxygenation parameters in the elderly and persons with spinal cord injury (SCI) changed significantly during the loading period (p < 0.01) and took longer time to return to the normal level than the health persons. TOI in the three groups decreased with the pressure and returned after pressure release. These findings indicated that pressure had significant effect on blood oxygenation and the oxygenation parameters are good indicators of evaluation of pressure sore risk.