The non-uniform x-ray line broadening in goethite-derived haematite has been investigated by Rietveld structure refinements and transmission electron microscopy (TEM). Mechanisms reported in the literature for this phenomenon are critically discussed. Based on the structure refinements of whole x-ray diffraction patterns and TEM measurements of various goethite-derived haematites, we demonstrate that the non-uniform x-ray line broadening effect is caused by the platelike shape of the haematite crystals.