The accuracy of 20-MHz Doppler catheter coronary artery velocimetry (Millar Instruments) was evaluated by model experiments using a turntable and a flow tube. In the turntable experiments, the flow away from the catheter tip (away flow) was significantly lower than that toward the catheter (toward flow), but the measured velocities for both away and toward flows were always lower than the known velocities. Nevertheless, the blood velocities measured at a sampling point 4 mm from the catheter tip showed excellent correlation with the known velocities irrespective of flow direction. The shape of the FFT spectra of the Doppler shift frequency from blood was broadened, and the frequency corresponding to the known velocity was always close to the maximum Doppler shift frequency. We concluded that zero-cross detection of blood velocity by the 20-MHz pulsed Doppler catheter provides a reliable measure for evaluating relative velocity changes, although it underestimates the blood velocity.