In this study, a method was proposed to change the surface roughness of Polydimethylsiloxane (PDMS) membrane, a hydrophobic material coated on circular steel substrates, using ultrasonic vibration with frequency above 20 kHz to control the surface tension of water droplet on the PDMS membrane. Ultrasonic vibration applied from piezo actuator was transmitted through circular steel substrate to PDMS membrane and it made the PDMS (liquid) vibrated. It made surface roughness of PDMS membrane changed. The results of coating the PDMS membrane on the circular steel substrate and measuring the contact angle of the water droplet after applying ultrasonic frequency vibration showed that ultrasonic frequency vibration could affect the surface roughness of the PDMS membrane. In this paper, we compared and discussed the surface roughness effects on various vibration frequencies and amplitudes.