In high-frequency swept-scan Doppler system, the spectral broadening of tissue clutter limits the detectability of low-velocity flow signal. Conventionally, the scanning speed of transducer has to be reduced to alleviate the clutter interference but at the cost of imaging frame rate. With third harmonic (3ƒ0) transmit phasing, the tissue harmonic clutter is suppressed and the cut-off frequency of wall filtering can be reduced to preserve low-velocity flow without compromising the frame rate. Our results indicate that the 3ƒ0 transmit phasing effectively reduces the harmonic clutter magnitude and thus improves the flow signal-to-clutter ratio. Compared to the conventional counterpart, the clutter-suppressed color flow and power Doppler images show fewer clutter artifacts and is capable of detecting more low-velocity flow of microbubbles. The resultant color-pixel-density also improves with clutter suppression. For the swept-scan high-frequency system, 3ƒ0 transmit phasing is capable of providing effective clutter suppression. With the same achievable scanning speed, the resultant Doppler image has higher sensitivity for low-velocity flow and is less susceptible to clutter artifacts.