A digital holography based method for online measurement of crystal size distribution is presented. The main advantage of holography over conventional imaging techniques lies in its capability to measure the three-dimensional volume information, thereby overcoming limitations associated with the depth of focus. In the proposed method, 3D volume information (holography) is stored and tailored image analysis routines are used to extract the size and shape information from the stored holograms. The method is applied to experimental batch cooling crystallization of two systems, namely, oxalic acid and potash alum. The measured crystal size and shape distribution are found to be in good agreement with offline microscopy measurements.