Cerebral Blood Flow (CBF) is one of the main parameters that reveal the brain function. In this paper, fundamentals for the measurement of CBF with near infrared spectroscopy (NIRS) were introduced. Results of relative changes of blood flow in a brain model were measured by a dual wavelength NIRS system that has specifications of real time sampling, data processing and displaying. An in vivo measurement of relative changes of blood flow in frontal cortex was reported.