This research was carried out to grasp the factors influencing to the ultrasonic velocity to improve the efficiency of the ultrasonic wave test using as the method of NDE of wood. However, it was difficult to detect the defect of wood accurately, because ultrasonic velocity is influenced by various factors. So, some effect factors, which were not concerned with decay, were confirmed at first. Therefore, in this research, we tried to find the factors which affect on the natural characters and the ultrasonic velocity at decayed wood, additionally, we tried an anatomic experiment and analyze. As a result of research, the velocity at radial direction was faster than that at tangential direction and the ultrasonic velocity at immature wood and reaction wood was slower than that at normal wood. And the ultrasonic velocity was more the slower at bigger the length of decay and reduction of weight.