A pattern-fitting procedure was described to determine the strength of preferred orientation of crystallite in tablets using X-ray powder diffraction data. This method is based on the pattern-fitting crystal structure refinement method which also uses powder diffraction data. Observed X-ray scattering intensities were fitted to analytic expression including some fitting parameters, namely peak positions, peak widths, background intensities and preferred orientation. All fitting parameters were optimized by non-linear least-squares procedure. Then the strength of preferred orientation of crystallites was evaluated from the magnitude of preferred orientation parameter.Aspirin was used for the sample, and though aspirin crystallites showed little preferred orientation in powders, they showed marked preference in tablets. The magnitude of preferred orientation parameter increased with increasing compression pressure of tablet. This indicated that aspirin crystallites showed stronger preferred orientation in tablets prepared under higher compression pressure.