In a floating gate semiconductor nanostructure biosensor and a method for manufacturing the biosensor, the nanostructure biosensor includes a substrate, an insulating layer, a nanostructure, a source electrode and a drain electrode, a floating gate and a biological sensing material. The insulating layer is formed on the substrate. The nanostructure is protruded from the insulating layer. The source electrode and the drain electrode are formed on the insulating layer and dispose the nanostructure therebetween. The floating gate has a metal pattern or a polysilicon pattern, and extends with contacting the nanostructure. The biological sensing material has a first end combined with an immobile molecule on the floating gate, and a second end combined with a bio molecule.