The optimal operation of a continuous crystallizer is studied by taking a real industrial scale crystallizer as an example. The objective function to be maximized is the production rate of the crystals which are larger than a certain size. Since continuous crystallizers exhibit sustained oscillation, it is important to find the optimal operation under oscillatory condition. In this study, the optimal operating condition is derived for two types of operation: one is the case where the manipulated variables are kept constant (constant input operation), and the other is the case where the values of manipulated variables are changed periodically (time varying input operation). The optimization result of time varying input operation indicates that the production rate of large crystals can be increased if the number of fine crystals in the vessel is reduced by withdrawing along with the product. Furthermore, this study examines the optimal operation of the crystallizer in the case where the sustained oscillation is eliminated by a stabilizing controller. The optimization result clearly indicates that the production rate of large crystals can be significantly increased if the operation is optimized under the condition where crystal size distribution (CSD) is stabilized.