Abstract: A novel scheme was proposed for identifying and extracting vortex structures from two-dimensional velocity fields. The new scheme involves no threshold values and requires a lower spatial resolution than velocity-gradient based methods. The scheme was used to analyze the characteristics of vortex structures based on two-dimensional velocity data measured with particle image velocimetry in a turbulent open channel flow. The educed parameters show that population density and vorticity of spanwise vortex decrease along the wall-normal direction while the radius tend to increase all the way. In addition, vortex strength represented by the circulation stabilizes after a quick increase in the inner region. Comparison demonstrates that population density of the prograde vortex is larger than that of the retrograde one and the prograde vortices exhibit stronger swirl motion. Conditionally averaged velocity fields of vortical structures statistically verify the existence of hairpin vortex packets and the spatial correlation of retrograde-prograde vortices in open channel turbulence.