The effect of thickness-to-chord (t/C) ratio on flow structure of a delta wing with sweep angle of 35 deg is characterized in a low-speed wind tunnel using laser-illuminated smoke visualization, particle image velocimetry, and surface pressure measurements. Four different t/C ratios varying from 4.75 to 19% are tested at angles of attack, 4, 6, 8, and 10 deg, for Reynolds numbers Re =1 x 10(4) and 3.5 x 10(4). The results indicate that the effect of thicknessto-chord ratio on flow structure is quite substantial, such that, as the wing thickness increases, the flow structure transforms from leading edge vortex to three-dimensional separated flow regime. The wing with lowest t/C ratio of 4.75% has pronounced surface separation at significantly higher angle of attack compared with the wing with highest t/C ratio, which indicates that lowest t/C ratio wing might be more resistive to the stall condition. However, considering the low angles of attack where all wings experience leading edge vortex structure, the strength of the vortex structure increases as the t/C ratio increases, which might suggest a better vortex-induced lift performance with high t/C ratio wing at low angles of attack.