Abstract: A new method for in situ measurement of hemoglobin oxygen saturation (SO2) in brain tissue by the combination of the spectrometer and fiber probe is presented. In situ visible absorption spectrum measured by the fiber optical spectrometer at different SO2 and their corresponding SO2 values measured by OXI meter was collected on a kind of tissue model, which was a mixture of Intralipid and blood. The spectra were normalized and four spectra characters were obtained between 500 and 600nm. An empirical formula of the relationship between the four characters and hemoglobin oxygen saturation is derived by statistical method. SO2 values of 30 living ratsbrain cortex at different depth were obtained, and the experimental range was 64%～76%. This method can be used for monitoring hemoglobin oxygen saturation in different depth in brain Minimally Invasive Surgery.