The change of the concentrations of Hb (deoxygenated hemoglobin),HbO 2(oxygenated hemoglobin) in tissue can be qua ntitatively determined by the record of near infrared absorption spectroscopy o f hemoglobin under different physiology status.The effect of the overlaying tiss ue on the measurement can not be ignored because the most interested tissue to b e monitored is the deeper tissue layers,such as muscle and brain,which are under adipose or skull.Apart from this,variations of the regional hemoglobin oxygen s aturation (rs O 2),haematocrit and regional blood volume hav e their influences on the measurement.To delineate the role of these factors in tissue oxygen moni tor,a multi-layered Monte-Carlo method was applied in this paper.The result of the simulation shows the accuracy of tissue oximeter decreases with the increas e of the thickness of overlaying tissue;The higher variation of SO 2 itself,th e higher regional blood volume in tissue and the higher haematocrit in blood int roduce additional error in measurement. -