The primary aim of this study was to investigate the enhancement effect of low-frequency ultrasound on skin permeation. In vitro permeation experiments were performed using Franz modified diffusion cells with ketoprofen as model drug. The effect of various ultrasound factors-ultrasound application mode (continuous mode and discontinuous mode), ultrasound intensity (0.26 $W/cm^2$, and 0.29 $W/cm^2$) and duty cycle (3%, 16%, 50%, and 83%) were studied. The highest permeation was observed at 0.29 $W/cm^2$ intensity, 50% duty cycle, and discontinuous mode. The result suggested the feasibility of low frequency ultrasound application for the phonophoretic transdermal drug delivery system.