An off-surface flow visualization experiments have been performed to investigate the flow field over a delta wing with the leading edge extension(LEX). The model is a flat wing with $65^{circ}$ sweepback angle. The free stream velocity is 6.2 m/s, which corresponds to Reynolds number of $4.4 imes10^5$ based on the wing root chord. The angle of attack and sideslip angle range from $16^{circ}sim28^{circ}$ and $0^{circ}sim-15^{circ}$, respectively. The visualization technique of using the micro water-droplet and the laser beam sheet enabled to observe the vortical flow structures, which can not be obtained by 5-hole probe measurements. With sideslip angle, the interaction and breakdown of the LEX and wing vortices was promoted in the windward side, whereas, it was suppressed in the leeward side.