The influence of hematocrit on flow velocity measurement by the ultrasonic Doppler method was investigated with an experimental blood flow system, which was composed of a silicone tube 7 mm in diameter and a roller pump. Flow volume was measured by the messcylinder method, and mean flow velocity was calculated from the flow volume divided by the cross sectional area of the tube. Mean flow velocity was also determined by the pulsed Doppler method, using a 5 MHz sector scanner, an axial size of sample volume of 10 mm, a low cut filter from 100 Hz to 400 Hz, and an instantaneous mean velocity display. Results from the preliminary study in the starch-glycerine solution showed that responsible factors for errors were the size of the low cut filter and the viscosity of solution, but not the number of particles of ultrasound reflectors. In hematocrits of 34%, 46%, and 52%, mean flow velocity was almost equal between the two methods. However, in abnormally high hematocrit (60%, 65%, 74%), mean velocity measured by the Doppler method was overestimated by about 15% in the high flow velocity range, and in low hematocrit (9%, 20%), it was underestimated by about 10%. Thus, abnormally high or low hematocrit may cause errors in mean flow velocity measurement by Doppler ultrasound.