The paper examines the influence of pulsed ultrasound action on nucleation kinetics in batch crystallization of borax in a crystallizer with controlled cooling. All tests were performed in a laboratory scale crystallizer with a volume of 2,67 dm 3 . The mother liquor saturated at 30 °C was cooled at a rate of 6 °C h −1 . The first part of the experiment concerned on the implementation of crystallization without the use of ultrasound. The mother liquor was then treated simultaneously with mechanical stirring and ultrasound. The tests were performed at impeller speed that ensured a complete suspension state of crystals in the system and with an ultrasound amplitude of 20 % where four different pulsed ultrasound actions were used (20, 50, 70, 100 %). Experimental results indicate that the application of ultrasound in the system leads to earlier nucleation. Under all analyzed conditions, nucleation takes place by the mechanism of primary heterogeneous nucleation, and its rate, as well as the metastable zone width, decreases with increasing pulsed ultrasound action. Intensifying the pulsed ultrasound irradiation also reduces the weight mean diameter of the final crystallization product and narrows it is span, especially in comparison with a system without ultrasound. Under the analyzed process conditions, the intensity of pulse action of 𝑝 = 50 % proved to be the most suitable option since the crystals obtained under these conditions are extremely regular in shape, apropos the sonofragmentation effect is not as pronounced as at higher pulse values.