In order to investigate the influence of liquid viscosity and surface roughness of the substrate on the improvement of wettability by ultrasonic vibration, a liquid droplet was put on a Langevin type vibrator, and ultrasonic vibration was applied to observe the change of the droplet shape. The droplets were deformed by the application of ultrasonic vibration, and the contact angle between solid and liquid was reduced, so that the wettability was improved. It was considered that the ultrasonic radiation pressure acting inside the droplet had an effect on the deformation of the droplet, and the value of the radiation pressure was estimated based on Laplace’s equation. It was confirmed that when the viscosity of the liquid was high, the change in the shape of the droplet was prevented by an increase in the shear stress for deformation. Regarding the surface roughness, it was found that the pinning effect made it difficult to reduce the contact angle. When the ultrasonic vibration was stopped, the shape of the droplet recovered to some extent before the ultrasonic vibration, but did not completely return to the original shape.