The aim of this study was to examine the influence of impeller size in the crystallizer with and without the ultrasound irradiation on crystal growth of borax during batch crystallization at the critical impeller speed (N = N JS ). Crystallization is carried out in a laboratory scale crystallizer by cooling the mother liquor saturated at 30 °C to a temperature of 15 °C, at the rate of 6 °C h -1 . A radial straight four blade turbine (abbrev. SBT) was used in the work, and the influence of impeller size on the formed crystals properties was examined at impeller to crystallizer diameter ratios, D/d T , of 0.43, 0.53 and 0.63, respectively. The same examinations were conducted in a crystallizer with an ultrasound irradiation at amplitude of 20 %, as in a silent crystallizer as well. During the experiments, a granulometric analysis of the growing crystals was performed by the laser diffraction method, and influence of the SBT impeller diameter on the changes of volume density crystal size distribution over process time was analysed in the system with and without ultrasound. The changes of the volume mean crystal diameter and its coefficient of variation over process time were also determined. In order to gain insight into the influence of hydrodynamic conditions in the crystallizer on the growing crystals habit and to detect the appearance of crystal agglomeration and secondary nucleation, photographs of crystals analysed with an optical microscope as well as their specific surface area were analysed respectively. The granulometric analysis of the final product revealed that the average crystal size in both systems increased with an increase in impeller size. In the silent systems, the proportion of finer and coarser crystals in the product also increased due to increased secondary nucleation and agglomeration. In the irradiated systems agglomeration was avoided, while the crystals were more than twice as smaller, but of a more regular shape. The power consumption due to stirring and ultrasound irradiation was also determined. In in both systems it slightly increased with impeller size, but in the system with ultrasound it was almost fifteen times higher.