Congenital heart defect is a leading cause of children mortality. Two-dimensional echocardiography is a safe and noninvasive diagnostic tool and two-dimensional images can be reconstructed to a three-dimensional model. However, for some complex congenital heart diseases, the visualization of three-dimensional cardiac tissue structure alone is not enough. Visualization of blood flow patterns in a human heart is important to evaluate cardiac disease of patients. Color flow imaging is a well-established ultrasound mode and very valuable for visualizing the distribution of blood flow in a specific region of interest. However, blood flow patterns in a human heart have three-dimensional complex structure and change dynamically, but the output of color flow imaging is usually presented as a two-dimensional image on a cross section of a heart. The authors make use of an echocardiography system to acquire the data which include cardiac tissue structure and blood flow patterns, and construct an interactive visualization system for the three-dimensional heart and blood flow. With the visualization system, which possesses interactivity and a changeable view, users can easily understand the inner cardiac structure and blood flow in the heart. The images that we use are not only from actual patient data, but also from isolated porcine hearts and hearts of living dogs.