Holographic particle image velocimetry (HPIV) encounters problems such as intrinsic speckle noise, external noise and aberration, and practical restrictions in test flow volume or media. The novel technique, In-line recording off-axis viewing (IROV), uses a single laser beam to record a hologram which is then viewed off axis during reconstruction. It combines advantages of both in-line and off-axis holography, achieving high spatial information capacity, low noise, and practicality. Instantaneous velocity vector field in the 3D flow of an unstable vortex ring in water has been measured by IROV. The vorticity distribution and circulation as a function of radius from the vortex core center are extracted from the 3D velocity field.