Spectral domain optical Doppler tomography (ODT) can achieve much higher imaging speed and velocity dynamic range than the time domain technology. A method to measure the absolute velocity is developed in a spectral domain ODT system,where Doppler angle is determined by combined information on the measured Doppler shift and Doppler bandwidth,and then the flow velocity is determined from Doppler angle and Doppler shift. Theoretical development relevant to Doppler measurement and absolute velocity determination is given. Measurement on flow velocity of aqueous solution of polystyrene beads in a capillary tube is done,in good agreement with the expected one. Finally,the developed method is applied to the study of rat’s cerebral arteries.