X-ray diffraction (XRD) studies were performed on carbon nanotubes (CNTs) with various alignment degrees synthesized by catalytic pyrolysis of ferrocene and xylene. A qualitative relationship between the peak intensities of the XRD patterns and the degrees of nanotube alignments was established. The intensity of the (002) peak decreases monotonically with higher nanotube alignment, This method provides a simple way to characterize the alignment degree of as-grown large-area CNTs.