A continuous-discrete observer was designed to estimate the CSD (Crystal Size Distribution) in batch crystallization processes. The observer is based on the discretization of PBE (population balance equations), it describes the evolution of the CSD with the finite difference method. Since the discretization leads a high order model, a reduced model was then constructed in order to simplify the observer synthesis and to reduce the computation time. The observer’s output allows to calculate the moments of the CSD, which may be used for control purposes or process supervision.