The evaluation algorithm employed in laser diffraction spectroscopy is usually based on the Fraunhofer diffraction theory for spheres. As this evaluation theory is limited to spheres only, the measurement of irregularly shaped particles results in errors in the evaluated particle size distribution. To determine the influence of the particle shape, measurements of arbitrarily shaped particles were simulated, evaluated and compared with results for spherical particles of the same projected area. The detailed investigations showed that the particle＇s microstructure, i.e. its surface roughness, has almost no or only a negligible effect on the measurement result. In contrast to this advantageous outcome, the particle＇s microstructure, i.e. its axis ratio, affects the measurement result greatly and must be taken into account in the evaluation method. Based on these results, particles were considered to be ellipsoids with specific (but negligible) surface structures and a mathematical adaptation method was developed by extending the evaluation theory from spheres to randomly in space oriented ellipsoids. Simulations and experiments with reticles verified the need for this adaptation technique and its efficiency when applied.