In order to reduce the energy loss resulting from streamline curvature and to enhance the efficiency of hydrodynamic torque converters. A new design method, three dimensional design method based entirely on circular arcs and straight line segments, was proposed. General Cartesian equations of three dimensional streamlines were established and general parametric equations were derived, a program code was developed, and the feasibility of the method was verified. The investigation result shows that the three dimensional design method based entirely on circular arcs and straight line segments has following advantages: firstly, the curvature radius of each three dimensional streamline reaches its maximum and remains constant, which will significantly reduce the energy loss resulting from streamline curvature. Secondly, near the core side, each streamline at the blade exit is designed into a short three dimensional straight line segment, which is helpful in reducing the flow deviation angle. Therefore, the energy loss caused by flow deviation can be reduced. That is to say, for a torque converter designed in this method, its peak efficiency can reach a maximum. In addition, since blade curvature and skewness are substantially low, and each blade only possesses twodimensional curvature, it is relatively easy for a torque converter to be fabricated. 