Vector Flow Mapping (VFM) is a new method for measuring blood flow vector and is based on Echo-Dynamography which measures various cardiac functions [Ohtsuki S. and Tanaka M., Journal of Visualization, Vol.9, No.1, 69-82 (2006)]. By using VFM, we can measure speed and direction of blood flow by estimating the velocity component in the direction which is perpendicular to the direction of the ultrasound beam. This velocity component is estimated from color Doppler data which is the velocity component in the direction of the ultrasound beam. To evaluate the accuracy of VFM, the fluid flow simulation software (PHOENICS, CHAM, UK) was used to generate both known flow velocity distributions and a 3D flow model. Furthermore, the 2D flow velocity distribution and the color Doppler data on the observation plane were calculated from the 3D flow model. The 2D flow velocity distribution was also estimated by VFM using the color Doppler data. The 2D velocity distribution estimated by VFM was compared very well with the 2D velocity distribution reconstructed from the flow model. In conclusion, it is concluded that reliability of this new method is very high. The results suggest that VFM has great potential for quantitative diagnosis of cardiac functions. We are currently evaluating the clinical usefulness of VFM.