The aim of this study was to observe the effect of ultrasound on particle size and molecular weight of whey proteins. In this work high-intensity ultrasound (20kHz probe and 40kHz bath) were used. 10wt.% protein model suspensions of whey protein isolate (WPI) and whey protein concentrate (WPC-60) were treated with ultrasound probe (20kHz for 15 and 30min) and ultrasound bath (40kHz for 15 and 30min). The results of particle size distribution have shown that, after treatment with an ultrasonic probe of 20kHz, ultrasound caused a decrease in particle size, narrowed their distribution, and significantly increased the specific free surface in all samples. After treatment with ultrasonic bath of 40kHz, there was a significant reduction in the size of particles. After treatment with probe of 20kHz there was a significant decrease in molecular weight and protein fractionation. Ultrasonic bath treatment with 40kHz ultrasound also showed significant changes in the composition of the molecular weight of protein fractions. Prolonged treatment of WPI with ultrasonic bath of 40kHz encourages the formation of aggregates of molecules.