Abstract: In this paper, we discuss a method for designing an air damper composed of a piston and a cylinder. When we design an air damper, we usually attend to obtain heavy damping. The amount of damping in the air damper, however, is limited by the compressibility of air. In an incompressible fluid damper such as the oil damper, the damping would increase with an increase in the length of the piston and a decrease in the clearance between the piston and the cylinder. In the air damper, the maximum damping is achived at a finite piston length and clearance Infinite length of the piston and zero clearance produce no damping, because the air does not flow through the clearance. This paper presents a simple expression for calculation of the optimum piston length and clearance. The optimum design conditions of the air damper are tested on a vibratory model. The experiments showed that the present expression is useful for designing air dampers.