Abstract: The displacement thickness of boundary layer for lowspeed wind tunnel contraction is investigated by solving incompressible Reynoldsaveraged NavierStokes equations. The development of displacement thickness, the figures and tables for contraction of different size, contraction ratio and length are analysed. Numberical methods show that the flow field qulity of the wind tunnel could be improved remarkably by boundary layer correction of the contraction. The results indicate that the boundary layer of the contraction is at first thick, then thin, and thick again. When doing boundary layer correction, it is possible to do it only for the second half of the contraction without any adverse effect on the result. For a same contraction ratio, one boundary layer thickness distribution could be used for different contraction profiles with satisfying results. The result for axissymmetrical contraction is also applicable for rectangle one.