Abstract: Ultrasound-based measurement of relative blood volume can be used to assess patient's dry weight during hemodialysis. The results of relative blood volume were calculated from the ultrasonic transmission time measurement in the arteriol pot, and the accuracy of transmission time measurement is directly related to the reliability of the results of relative blood volume. There are various factors which influence the travel time, this article analyzed patients themselves, measuring device and the external factors, and advised appropriate counter measures.