Near infrared spectroscopy (NIRS) is a prospective non invasive technique for brain research.There exist some uncertainties about the volume and the depth of the NIRS can investigate.This paper presents some simulation results on a multi layer model for optical measurement on head by Monte Carlo method.Results show that the optically clear layer of cerebrospinal fluid greatly influences the light transport in head tissues.It permits the NIRS to obtain information from a deeper site but with a poor spatial resolution.