The development and interaction of vortices over a delta wing with leading edge extension (LEX) was investigated through off-surface flow visualization using micro water droplets and a laser beam sheet. Angles of attack of $20^{circ}$ and 24$^{circ}$ were tested at sideslip angles of $0^{circ}$, $-5^{circ}$, and $-10^{circ}$ The flow Reynolds number based on the main-wing root chord was $1.82{ imes}10^{5}$. The wing vortex and the LEX vortex coiled around each other while maintaining comparable strength and identity at a zero sideslip. The increase of angle of attack intensified the coiling and shifted the cores of the wing and LEX vortices inboard and upward. By sideslip, the coiling, the merging and the diffusion of the wing and LEX vortices were increased on the windward side, whereas they were delayed significantly on the leeward side. The present study confirmed that the sideslip angle had a profound effect on the vortex structure and interaction of a delta wing with LEX, which characterized the vortex-induced aerodynamic load.