Cancer cell morphological, geometric and mechanic characterization could be important in relationship to therapy modality used. The objective of this work was to analyze the correlation between cell size and low intensity ultrasound sonication frequency for attached and floating cells. MCF7 and U937 cells were stressed by ultrasound in a range between 400 kHz and 620 kHz at 10 Hz PRF (Pulse Repetition Frequency) for three minutes. Immediately after sonication, the vitality and the diameter were evaluated. Although sensitivity to frequency is more evident for attached cells, some effect can be clearly detected also for floating cells: it seems that each fixed frequency prefers to kill floating cell with a particular value of diameter, with a maximum value of killing rate. The results obtained in this research work support the frequency plays a significant role on cell treatment. This result could be interesting not only to explain the proven ultrasound selectivity but also to destroy cancer cells, preserving healthy cells.