Many methods have been developed to measure crystal particle size. The online Focused Beam Reflectance Measurement (FBRM) and a classical offline Laser Diffraction Method (LDM) were used to measure the size of the polyvinylchloride (PVC) particles and hydrocortisone (HC) crystals. For the standard spherical PVC particles, the particle sizes measured using both FBRM and LDM are similar. However, for the anomalistic HC particles, the difference between the particle sizes measured by the above two methods increased with the increase in the degree of scrambling. The online monitoring of the particle size by direct installation of a FBRM probe into the crystallization process helped indicate the crystal quality, and thus, improve the control of crystallization processes.