Abstract: Large-scale spiral tail vortices at the outlet of a radial hydraulic turbine would affect the efficiency and stability of the machine. To reveal the flow mechanism, a Q criterion is used to identify and extract the structure of different tail vortices, focusing on analysis of velocity fields. The results show that the vortices rotate opposite to the impeller with their core radius about one third of the impeller outlet radius. We divide a tail vortex into a core zone and a decay zone by the radial profiles of its tangential velocity at the impeller outlet. Transversely, the tangential velocity increases sharply along the radius in the core zone, and peaks at the core radius; then it decreases gradually in the decay zone down to a level of roughly half the peak at the tube wall. Along the flow, its variation flattens out gradually and its peak moves gradually toward the channel wall. We find the vortex core is a zone of energy loss dominated by the kinetic energy loss of tangential velocity, which is one of the major causes of energy loss in a radial hydraulic turbine.