A transistor using a single crystal silicon nanowire and a method for fabricating the transistor is disclosed. The transistor using a single crystal silicon nanowire comprises a substrate and a single crystal silicon nanowire formed on the substrate. Here, the single crystal silicon nanowire comprises a source region and a drain region formed longitudinally with the single crystal silicon nanowire and separate from each other, and a channel region located between the source region and the drain region, wherein the perpendicular thickness of the channel region to the longitudinal direction is thinner than the thickness of the source region and the drain region.