Effects of blade thickness on discrete frequency noise (DFN) have been investigated using symmetrical NACA blades of three different thicknesses. It was clarified that the mechanism of DFN generation was different depending on the thickness of the blade. In the case of a thin blade, NACA 0008, the air flow was separated upstream of the trailing edge on the suction side of the blade, but was attached to the trailing edge of the blade on the pressure side. Therefore the frequency and the sound pressure level of DFN were varied with the variation of the attack angle. On the other hand, in the case of a thick blade, NACA 0015 or NACA 0018, the flow was separated upstream of the trailing edge on the pressure side while it was attached to the trailing edge on the suction side. The frequency and sound pressure level of DFN did not vary with the variation of the attack angle in these cases. The location of the rolling up of Karman vortices was very important in the generation of DFN. When rolling up of the Karman vortex took place far from the trailing edge of the blade, the Karman vortices did not have a strong effect on the flow around the blade. However, when Karman vortices rolled up in the vicinity of the blade, the interaction of the Karman vortices with the blade was strong, causing DFN generation.