One of the most important problems in an oil hydraulic system is the contamination problem. The aim of this study is to evaluate numerically the contamination of an oil. The measurement is carried out to evaluate a degraded oil using the ultrasonic sensor. As the ultrasonic wave propagates through a degraded oil, the energy of the ultrasound is scattered by the contaminant particle, and hence the some attenuation is occurred. This indicated that the attenuation of ultrasonic wave is closely connected with the distribution of each contamination particles.