Generally, industrial vehicles such as agricultural machines and construction machinery use hydraulic systems for heavy-load work. Also the work often continues for a long time in a certain area. Consequently, noise exposure of the surrounding environment is increased. Thus noise regulations are established in several countries. The noise emitted from the hydraulic system has a significant contribution to the over-all of the noise in an industrial vehicle. Recently, the choice of power sources in industrial vehicles has tended to shift from internal combustion engines to electric motors. Consequently, the contribution of noise emitted from the internal combustion engine will decrease and the contribution of noise emitted from a hydraulic system will become relatively prominent in the future. It is, therefore, more than ever important to take care of the noise generated by the hydraulic system of the industrial vehicles. It is known that speed of sound in hydraulic oil has an effect on pressure ripples which can be source of noise. Therefore, it is important to understand behavior of the speed of sound reasonably in design of quiet hydraulic systems. However, relationship between operating conditions and the speed of sound has not been revealed up to now. In this study, the speed of sound has been investigated at the operating conditions that are oil temperature, loading pressure, volume ratio of entrained air to the hydraulic oil and type of the oil for typical agricultural machines and construction machinery.