A method was developed to quickly evaluate the streak line pattern in a laminar mixing vessel by numerical analysis. The primary flow in a laminar mixing vessel is the rotational flow, but the secondary flow as the upper and lower circulation flow determines the quality of the mixing. The three-dimensional steady velocity component of r-theta-z was regarded as the two-dimensional non-steady velocity component of r-z-t. As a result, the calculated streak line pattern agreed qualitatively well with the experimental pattern, and calculating time was reduced substantially. By using the present method to translate the three-dimensional steady flow to a two-dimensional unsteady flow, streak line patterns were easily visualized from video data with PIV at low Reynolds number. It was found that the mixing performance of impellers, including some wide-paddle impellers developed by a Japanese company, could be estimated quickly by this method.