A novel way for converting crystal mean chord length (MCL) to the mean size (MS) directly was applied in this paper. Using focused beam reflectance measurement (FBRM) technique, the crystal growth process of phosphoric acid hemihydrate was investigated in cooling mode. In order to convert mean chord length to mean size and verify reliability of FBRM data, digital photo-technique in real time was applied in large crystal size range. The results indicated converting constant A between mean chord length and mean size was 7.7 and crystal growth was acted on size-independent growth (Mcabe's Delta L law). Combining FBRM in real time with on-line digital photo-technique, the reasons for appearing turn point during phosphoric acid crystallization process were proved to be crystals breakage and agglomeration. The crystal growth rates under different relative supersaturations were determined and the order of crystal growth obtained from the exponent function is 1.07. It can state that the growth of phosphoric acid hemihydrate crystal was mainly controlled by diffusion. (C) 2009 Elsevier B.V. All rights reserved.