Doppler optical coherence tomography (OCT) is a noninvasive imaging modality that provides quantitative flow information with high spatial and temporal resolution. However, it is only sensitive to the flow velocity vector parallel to the incident beam. To calculate the absolute velocity, it is necessary to obtain the angle between the incident beam and flow field. In this paper, we describe a practical method to measure the Doppler angle based on the structural information of blood vessels extracted from spectral domain OCT images. In this method, a normal sectional scan of the vessel is performed where the probe beam is perpendicular to the vessel. Next, the axial diameter (Z direction) of the vessel (D-A) was measured in the acquired image. For a certain scan in which the probe beam is oblique to the blood vessel, the axial diameter of the vessel (D-A') can be measured. Thus, the Doppler angle can be calculated depending on the ratio of D-A and D-A', and absolute blood flow was determined. We validate this method in a capillary tube as well as in large blood vessels of early-stage chick embryos. This technique is suitable for early-stage embryo blood-flow measurement because most of the blood vessels are easily differentiated from the transparent surrounding structures during that time. (C) 2017 Optical Society of America.