Flutter constraint, applicable to aircraft conceptual design, is constructed using response surface methodology. It is presented by the critical flutter speed, as a function of wing torsion stiffness, root chord, sweep, mass ratio, taper ratio, aspect ratio, center of gravity location and radius of gyration. The constraint function to is a quadratic response surface polynomial. The D-optimal design is used to find the best combinations of design points required to determine the function coefficients. 'The Regier number criterion is used to calculate the critical flutter speed at these design points. Analysis of variance is used to remove the unreliable terms from the function. To match the Regier number criterion, two constraint functions suitable for subsonic aircraft with traditional wing are constructed. The first one is applicable to aircraft with low sweepback wing while the second one is applicable to aircraft with moderate sweepback wing. As a case study, the constraint function is applied within the conceptual design of a subsonic aircraft leading to a considerable weight saving.