Ultrasound-mediated gene transfection (sonotransfection) is a promising physical method for gene therapy, especially for cancer gene therapy. To investigate the optimal sonotransfection conditions and to determine whether the optimal transfection rate using sonotransfection is comparable to that of electrotransfection or liposome-mediated transfection, we sonicated different cancer cell lines (U937, HeLa, PC-3, Meth A and T-24) using a 1-MHz unfocused ultrasound at different intensities, pulse repetition frequencies and exposure times. The ideal ultrasound conditions were noted to be at 1.5 Watt/cm(2) pulsed at 0.5 Hz with a duty factor of 50%. The results showed that transfection rate increased with the number of pulses, and peaked between 10 and 15 pulses before it started to decline. Using such optimal conditions, we have shown that sonotransfection is superior to electrotransfection and liposome-mediated transfection at the fixed conditions used in the present study. These findings suggest that sonotransfection could be a better alternative to other non-viral methods (e.g. electroporation and liposome-mediated transfection) of gene transfection, particularly in cancer gene therapy.