Aligned double-walled carbon nanotube (DWNT) long ropes with a narrow diameter distribution were directly synthesized by sulfur-assisted floating catalytic decomposition of methane. The DWNT ropes are typically up to several centimeters in length and possess good alignment and high purity. High-resolution transmission electron microscopy (HRTEM) images and resonant Raman spectra revealed that the outer and inner tube diameters of the DWNTs are narrowly distributed in the range of 1.7-2.0 and 1.0-1.3 nm, respectively. Moreover, based on the resonant Raman measurements, the electronic properties of the two constituent tubes of the DWNTs were identified. The successful synthesis of such DWNTs opens the possibility for their fundamental studies and further applications as nanomechanical, nanooptical, and nanoelectronic devices.