In this paper we propose a method to characterize waveguide shims, which are being used in almost all calibration kits for waveguide connectors and also used as impedance standard in Thru-Reflect-Line (TRL) calibration, for their length and aperture width and height. Also given are preliminary uncertainty analyses for the electrical characterization of the length, actually the phase shift through the shim, for two calibration methods for the Vector Network Analyzer (VNA).