The orientational distribution of cellulose nanocrystals (CNCs) in a cellulose whisker (CW) was investigated by means of the X-ray diffraction of magnetically oriented samples of CWs. A cellulose sample (Whatman CF11) was hydrolyzed and fractionated to prepare three different CW samples with a size ranging from ca. 10 to 100 mu m. Each of the fractions that were suspended in a liquid matrix was aligned under a static or a rotating magnetic field, and the matrix was solidified to prepare magnetically oriented microcrystal arrays (MOMAs). Then, the MOMAs were investigated by X-ray diffraction measurements. By analysis of the diffraction patterns, it is concluded that the c-axes of the CNCs are uniaxially distributed within a CW and that the orientational order increases with decreasing CW size. The average anisotropic magnetic susceptibility of the CWs was expressed in terms of their size and of the X-ray azimuthal half width. Using these expressions, a correlation length for the orientation of CNCs in a CW was determined. The proposed method enables one to investigate the orientational order in a macrofiber at arbitrary scales by magnetically aligning the small fragments segmented from the macrofiber.