X-ray diffraction technique is one of the best methods to study polymorphism of fats systems. The only information authors usually report is the interplanar distances calculated from the angles at which diffraction lines appear; these clearly determine the polymorphic form. However, in many other fields using crystallography, X-ray patterns are also used to determine crystal size from the broadening of the diffraction line at half the line of maximum intensity. To investigate the meaning of the diffraction line width in fat systems and the relationship with other techniques widely used to determine crystal size, we crystallized a blend of 30–70% highmelting in low-melting milk fat fractions at two different cooling rates. The crystallization process was followed by polarized-light microscopy. Crystal size distribution was determined by analyzing 200 crystals.