Abstract: The key of swept wing passive laminarflow control based on leading-edge roughness elements array technique is thechoice of control wavelength. The author advanced the quickest wave theoryfrom the view of the physical mechanism of control wave and flowinteraction. The author investigated infinite swept wing optimal controlwavelength through solving three-dimension incompressible Orr-Sommerfeldequation by using Mack method. Numerical results validated 8\,mm is thecontrol wavelength under Saric's experimental condition by fixed wavelengthmethod. By using adaptive searching method the optimal control wavelengthsof this model under different conditions. Transition delaying was achievedthrough wind tunnel experiments. This paper also advanced the technique ofroughness elements array appended by silk-screen, and mixed solventsublimation method, which enriched the experimental means.