When oil production declines due to low displacement efficiency and/or adverse mobility ratio, enhanced oil recovery methods are required to improve the production. Application of high frequency sound waves (also known as ultrasonic waves) as an unconventional enhanced oil recovery method has been point of interest for some decades. However, despite number of researches on the applications of ultrasonic waves, the influencing mechanisms are not fully comprehended. The aim of this research is to experimentally investigate the effects of ultrasonic waves on recovery of waterflooding and to discuss the mechanisms involved in this process. Series of straight and ultrasonic stimulated waterflooding experiments were conducted on a long unconsolidated sand pack and the oil recovery was measured at the end of the experiments. Kerosene and vaseline were used as non wet phase in the system. In addition to that, a series of supplementary experiments were conducted using ultrasonic bath in order to enhance the understanding about contributing mechanisms. The recovery of waterflooding increased by 2–16% was observed as a result of ultrasonic stimulation. Emulsification and viscosity reduction were identified as main mechanisms affecting improvement of oil recovery by waterflooding. The results of this research are expected to enhance the insight about the mechanisms involved in application of high frequency waves and contribute in wider application of this method for enhanced oil recovery purposes.