To design a miniaturized centrifugal blood pump, the impeller internal diameter (ID), which is a circle diameter on the inner edge of the vane, is considered one of the important aspects. Hydraulic performance, hemolysis, and thrombogenicity were evaluated with different impeller IDs. Two impellers were fabricated with an outer diameter of 35 mm, of which 1 had an 8 mm ID impeller and the other had a 12 mm ID. These impellers were combined with 2 different housings in which the inlet port was eccentrically positioned 3.8 and 4.5 mm offset from the center. The hydraulic performance and hemolysis were evaluated in a mock circuit, and thrombogenicity was evaluated in a 2 day ex vivo study with each impeller housing combination. Both impellers required 3,000 rpm in the 3.8 mm offset inlet to attain 5 L/min against 100 mm Hg (left ventricular assist device condition). The 8 mm ID impeller required 3,200 rpm, and the 12 mm ID impeller required 3,100 rpm in the 4.5 mm offset housing. The normalized index of hemolysis was 0.0080 +/- 0.0048 g/100 L in the 8 mm ID impeller with the 3.8 mm offset and 0.022 +/- 0.018 g/100 L with 4.5 mm offset. The 12 mm ID impeller had 0.068 +/- 0.028 g/100 L with the 3.8 mm offset and 0.010 +/- 0.002 g/100 L with the 4.5 mm offset. After the 2 day ex vivo study, no blood clot was formed around the top bearing in all the pump heads. The 8 mm ID impeller with 3.8 mm offset inlet and the 12 mm ID impeller with the 4.5 mm offset had less hemolysis compared to the other pump heads that were subjected to 14 day ex vivo and 10 day in vivo studies. The 8 mm ID impeller with the 3.8 mm offset inlet had a blood clot around the top bearing after the 14 day ex vivo study. No thrombus was found around the top bearing of the 12 mm ID impeller with the 4.5 mm offset in the 10 day in vivo study. These results suggest that the ID does not greatly change the hydraulic performance of a small centrifugal blood pump. The proper combination of the impeller ID and inlet port offset obtains less hemolysis. The larger impeller ID is considered to have less thrombogenicity around the top bearing.