Blood side and dialysate side flow distributions play an important role in determining the optimal use of dialysis membrane in hemodialyzers for the removal of uremic solutes. In this article, we used two nonintrusive magnetic resonance imaging (MRI) techniques called the two-dimensional phase contrast (2DPC) and two-dimensional Fourier transform (2DFT) velocity imaging techniques to (1) study the effect of space yarns on the dialysate side flow distribution, (2) investigate the effect of flow baffle on the dialysate side flow distribution, and (3) characterize the blood side and dialysate side flow profiles of hemodialyzers with flow rates ranging from 200 to 1000 ml/min. We investigated two types of hollow fiber hemodialyzers: hemodialyzers A (with spacer yarns) and B (without spacer yarns). We used a 3 mmol cupric sulfate solution as the compartmental fluid and imaged five transverse cross sections of these hemodialyzers. The hemodialyzer with spacer yarns had a more uniform dialysate side spatial flow distribution than that without spacer yarns. In addition, the design of flow baffle in these hemodialyzers can be further improved to promote uniform dialysate side flow distribution, and the blood side flow had a fully developed laminar flow profile. Our experimental results showed that these velocity imaging techniques provide an innovative, nonintrusive tool for characterizing flow distribution in hemodialyzers.