A light emitting diode (LED) lamp tube includes a circuit board and a tubular enclosure. The circuit board is disposed in the enclosure such that the enclosure is divided into a non-illuminated portion and a light-receiving portion. The light-receiving portion has two light-condensing side sections corresponding respectively to two sides of the circuit board, and a light-diffusing middle section connected between the light-condensing side sections. An outer surface of the light-receiving portion has a smoothly varying curvature. The light-condensing side sections cooperate with the light-diffusing middle section to diffuse uniformly LED light transmitted from an outer surface of the enclosure.