Quantitative information on the X-ray diffraction line profiles for different shaped nanocrystallites, using nanocrystalline Cu as an example, are obtained by using the Debye scattering equation, which is useful for characterizing nanocrystallite. The apparent lattice parameter obtained from line positions by using the Bragg's law has a larger deviation from the actual value when the crystallite sizes are smaller than about 10nm. When the crystallites are larger than about 6nm, the full width at half maximum (FWHM) ratios of FWHMhkl to FWHM002 are nearly constants which can be used to identify the shape of the crystallites. The Scherrer constant depends not only on the shape of the crystallite but also on the Miller index of the line, the values of the Scherrer constant for different shaped nanocrystallites are given in the present paper.