Abstract: An improved microwave tritium labelled apparatus was designed. It consists of a special vaccum circuit in which tritium gas can be cycled by a centrifugal mini-blower through the labelling reaction chamber. In this system a method of tritium labelling utilizing microwave activation of flowing tritium gas was developed. Using 8-BrcAMP as raw material, especially when 8-BrcAMP was precipitated on a SiO_2 carried Pd catalyst, the tritium-bromine replacement reaction occurred. The activity yield of ~3H-CAMP is more than that of ~3H-cAMP prepared by microwave tritium labelling without tritium gas circulating. It is an improved microwave method for tritium labelling on definite position of molecules.