We applied two different flow visualization techniques to obtain detailed information on the inside flow of the diaphragm blood pump of our electrohydraulic total artificial heart system to determine the optimum washout effect that would result in better antithrombogenicity. Major orifice directions of the inflow and outflow Bjork-Shiley valves of the left blood pump were independently changed to create 17 varied patterns. The character and velocity of the main flow at the diaphragm-housing junction were acquired using a laser light sheet method with polyethylene tracers. Wall shear flow, a major factor governing washout in the blood pump, was estimated by a newly developed paint erosion method. In this method, quantitative evaluation for an index of washout effect was made by calculating the residual ratio of the paint on the blood pump inner surface at 30 sec of pumping. When a single circular flow was consistently observed by the laser light sheet method, the paint residual ratio become low, indicating washout was relatively good. At the lowest paint residual ratio, the center of the circular flow observed by the laser light sheet method was located at the geometric center of the blood chamber. In conclusion, the flow pattern inside the blood pump could be characterized by combined use of these two flow visualization techniques, and the significant role of circular flow in better washout was clarified.