Ultrasound effects on the motion of a falling sphere in water (i.e. Newtonian fluid) and in PolyAcrylAmide (PAA) solution (i.e. non-Newtonian shear-thinning fluid) are experimentally investigated via an image processing technique. The falling speed in water is confirmed to be almost irrespective of the frequency or amplitude in the applied ultrasound, while that in PAA solution is found to be higher with increasing the ultrasound intensity. In consideration of pseudo-plastic factors estimated by various velocity and length scales involved in the system, we consider that the mechanism to enhance the falling speed has the influence of viscosity in the acoustic boundary layer and its thickness.