Abstract: Based on the results of oil flow visualization tests the mechanism of side wall effect for airfoil testing is investigated.It is shown that the traditional theory and concept of side wall effect are suitable only for subcritical flow conditions.In supercritical cases, the manifestation of side wall effect is much more complicated, and its influence much more pronounced. It is shown in the present paper that the oil flow patterns on the airfoil fall into five types. The main features for each of them are described. The mechanism o?side wall effect is interpreted both for supercritical and subcritical conditions. A systematic exposition of the development of oil flow patterns is made. It is pointed out that the old fashioned "two dimensional wind tunnel" of solid side wall is improper for transonic airfoil testing. Wind tunnel of large width may reduce the side wall effect,but it is not a final solution.It is shown that even for tunnel width of 3.4 chordlength, there may still be evident of side wall effect in the middle section of the model. It is believed that proper application of suction may be a promising solution for the side wall problem.