A detailed measurement of the lateral vortex structure embedded in a wall boundary layer is presented to simulate the development process of the leakage vortex in the turbomachinery. The vortex was generated by a wall jet issuing perpendicularly to the mainstream from a slit located at the corner of the wall between two parallel ducts. The flow field was measured experimentally by a 5-hole probe utilizing a three-dimensional traversing device. Streamlines were traced to determine the leakage flow and through flow boundary, which not only explains the distribution of total pressure loss, vorticity, and other properties well but also clarify the flow structure in the vortex core. An angular velocity of the fluid particle was computed via eigenvalue of the velocity gradient tensor to represent the flow structure of the leakage vortex. The characteristic of the flow system is that the streamwise vortex is accompanied by two shear layers in which loss is almost generated. The vortex core forms a set of Bernoulli surface and is found to generate little loss in the vortex.