The method which was developed by the author for analysing a simple cylindrical cavity with radial vanes has now been extended to the case of an interdigital resonator. The effect of the interdigital anode is taken into account in terms of modes of high orders, which are present only in the immediate vicinity of the anode. An approximation to the stored energy associated with these modes can be obtained by considering the equivalent static case. The effect of the vanes is accounted for by considering trial fields which satisfy the boundary conditions at the vanes. The evaluation of frequency proceeds by iterative steps, which essentially involve matching fields across an assumed dividing surface passing through the tips of the vanes. To get successively better approximations to the field distribution, variational principle is used.