To analyze wall interference of a transonic wind tunnel, flows of two-dimensional airfoil with wind tunnel porous walls are computed by CFD and the new porous wall model. First, the computational results are validated by comparing with measurement. The porous wall model shows high accuracy enough to precisely simulate the wall interference. Effect of the wall interference is discussed by comparing two kinds of computational results with and without the wind tunnel wall. Wall interference by the porous wall is characterized by the downwash and blockage. Porous wall is effective for decreasing the blockage, which causes the acceleration around the airfoil. However, high porosity of the porous wall causes the large downwash, and it changes incidence angle of the airfoil. The Mokry's correction method for wall interference is verified by applying the correction to CFD results. The Mokry's method shows high accuracy in subsonic and no stall conditions, which meet the requirement of small perturbation potential equation. The accuracy is also high even if the model is large relative to the wind tunnel size. However, about 10 drag count error is caused under the transonic flow. The accuracy is rapidly reduced around the stall condition, and over 100 count error is caused.