The streak line visualization method developed by Inoue et al. (2009) is effective for evaluation of mixing performance in the laminar flow region. In the previous work, the mixing mechanism of large paddle impellers that can be used over a wide range of viscosity was clarified based on this method. In the present work, the geometry of a new impeller that has good mixing performance was optimized based on streak line observation. The new impeller has (1) simple geometry, (2) speedy mixing performance, and (3) stable operating characteristics.