Purpose: The maximum measurable velocity of blood flow in conventional Doppler color flow mapping is limited due to the aliasing phenomenon. This paper describes a novel method, called “wideband Doppler,” for alias-free estimation of high blood velocity. Method: The new method presented here utilizes the wideband echo information obtained by the transmission of a short ultrasound pulse with a wide frequency band. Received echoes are Fourier transformed and converted to the cross spectrum. The phase of the cross spectrum is a function of frequency, and the gradient of the fitted line of the phase corresponds to the average velocity of the targets. When the target velocity is greater than the Nyquist limit, a part of the phase of the cross spectrum is greater than ±π rad, and is wrapped in ±π rad. The wrapped phase is corrected using wideband echo information to avoid aliasing. Then the gradient of the corrected phase is estimated and converted to the target velocity. Result: The results of computer simulation showed that the range of velocity estimation could be expanded to three times that of the conventional method. Furthermore, good robustness against noise and the dispersion of target velocity was also confirmed. Conclusion: The proposed method has the ability to estimate blood velocity beyond the Nyquist limit. An alias-free color flow mapping system might be realized with this method.