Abstract: The passive intermodulation (PIM) characteristics of S-band waveguide to coaxial adapter are studied experimentally using PIM analyzer. Firstly, the prototype adapter is measured and it is found that its 3rd PIM level is about-56 dBm when the power of input carriers is 40 dBm. Secondly, the prototype adapter PIM performance is ascribed to the following two factors: the effect of nickel plate on the outer conductor of the coaxial connector and the metal contact effect between the tuning disk and the inner conductor of the connector. The explanation is verified through PIM measurement of modified prototype adapter. Lastly, considering the ordinary low PIM design rules, a low PIM adapter is designed and manufactured. The measurement result show that its 3rd order PIM is about -114 dBm when input carrier is 43 dBm. The developed adapter can be used in antenna feeding system or PIM test setup for waveguide flange.