A method for detecting polarizing direction of electromagnetic wave includes disposing a carbon nanotube structure in a vacuum environment, irradiating a surface of the carbon nanotube structure by an electromagnetic wave with a polarizing direction while rotating the carbon nanotube structure, and determining the polarizing direction of the electromagnetic wave according to change of the visible light emitted from the carbon nanotube structure. The carbon nanotube structure includes a plurality of carbon nanotubes arranged along a substantially same direction. The carbon nanotube structure can absorb the electromagnetic wave and emit a visible light. The rotating axis is substantially perpendicular to the surface of the carbon nanotube structure irradiated by the electromagnetic wave.