The effects of sideslip on the vortex over a delta wing was investigated experimentallu at a free strean velocity of 40 m/sec, corresponding to a Reynolds number of 1.76$ imes$$10^6$, based on the root chord. The angles of attack ranged from $16{^{circ}}$ to $28{^{circ}}$, and the sideslip angles treated were $0{^{circ}}$, $-10{^{circ}}$, and $-20{^{circ}}$. It was observed that the sideslip decreased the strengths of the vortices of both windward and leeward sides of the wing, and promoted the vortex breakdown on the windward side. At sideslip angle of $-10{^{circ}}$, the vortex strength of leeward side was increased as the angle of attack increased. This asymmetric development and breakdown of vortices in sideslip condition would cause an abrubt change of the rolling moment at a high angle of of attack, which could be considered as a rolling moment instability