This paper describes the princi ple and specification of the beam current transformer(BCT). Sensitivity is equal to 0.47V/A. Frequency response is great than 100MHz.Drop of the flat top is les s than 0.4%/μs.The beam current transformer has been used to measure macropulse electron beam currents produced by 30MeV electron linear a ccelerator and the macropulse beam current waveforms measured by BCT are also given.