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An ultrasound vector Doppler imaging is useful for detecting flow components normal to the ultrasound beam direction. However, the conventional vector Doppler imaging method suffers from the bias of the time interval between samples caused by the mismatch between transmit and receive directions during demodulation. In this paper, a new directional demodulation method, in which demodulation is performed with a modified sample interval depending on the receive beam steered angle to reduce the bias occurred in a conventional ultrasound vector Doppler imaging is presented. To evaluate the performance of the proposed directional demodulation method, the pre-beamformed radio-frequency (RF) data from in-vitro experiments were obtained using a commercial ultrasound system and a Doppler string phantom. The true flow velocity of the phantom was 0.3 m/s. The center frequency of 5 MHz and the pulse repetition frequency of 4 kHz were used for the experiments. Also, a 32-element sub-aperture on a 128-element 7.2-MHz linear array probe were used for emission and reception while changing the flow direction from -45 degrees to 0 degree by a step of 5 degrees. The proposed directional demodulation method successfully visualizes all flow directions. In addition, it lowers a bias on flow estimation compared to the conventional method (i.e., 0.0255±0.0516 m/s vs. 0.0248±0.0469 m/s of error of velocity, 2.4862±3.8911 degrees vs. 2.4857±3.5115 degrees of error of direction, respectively). These results indicate that the proposed directional demodulation method can enhances the accuracy in flow estimation for vector Doppler imaging.