The dye-injection flow visualisation technique was used to investigate the effect of the apex flap on the leading-edge vortex breakdown over a cropped 76/40 double delta wing. The angle of. attack of the experimental model varied from 20 to 40o, and tile length of the apex flap was 25%c and 50%c rcspcctivcly. By changing the angle of the apex flap, we found that the apex flap is an efficient method to control the leading edge vortex breakdown, and rbat there exists an angle of the apex flap at which the value of the leading edge vortex breakdown delay reaches maximum. Moreover, it is found that, for a< 28, the small apex flap is a better choice for delaying the vortex breakdown; for a> 28, the large apex flap is superior to the small one.