In chemical engineering processes in which crystal is produced, the strength of the substances plays a key role. In this study, micro-level measurements of the fracture strength of aggregated crystal were performed under various operating conditions of an MSMPR crystallizer. The attrition and the fragmentation mechanisms were considered from the fracture strength, the fracture energy and impact velocity calculated from micro-hardness properties. The results showed that these properties had different values between aggregated and non-aggregated crystals. Inclusion of mother liquor caused by aggregation was accounted for the decline of the crystal strength. According to these parameters based on microhardness, there was a possibility that the collision between aggregated crystal and impeller caused not only attrition but also fragmentation. (c) 2007 Published by Elsevier B.V.