The efficiency was evaluated of the bi-frequency ultrasonic cavitation action on the process of preparation of starch sizes for cotton yarns. The application of the ultrasonic energy of bi-frequency cavitation provides the possibility to prepare sizes without chemical reagents for starch splitting and to lead the size preparation at a lower temperature about 85оС. The proposed method allows one to prepare the size in an open bath without high vapor pressures, to reduce considerably the time of size preparation and to decrease the consumption of food staffs – the starch up to 30% and vegetable oil up to 50%.