The polarizabilities of an annular aperture in a thick conducting plane are obtained for electric and magnetic cases. The polarizabilities are important parameters to describe the waveguide coupling by apertures. The Hankel transform is applied to express the scattered and transmitted potentials. The problem is solved by a combination of the analytic and numerical methods. The resulting series solution is fast convergent and very efficient for numerical computation. The numerical calculations are performed to illustrate the behavior of polarizabilities in terms of the aperture size and conductor thickness.