The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer due to be flown in conjunction with AMSU-A on the TIROS-N series of satellites. The instrument provides two 'window' channels centered on 89 GHz and 150 GHz and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. AMSU-B is specified to have an antenna pattern half power beamwidth (HPBW) of 1.1$DEG +/- 10%, beam efficiency of >=95% over the main beam (2.5 times HPBW) and to control mispointing of the antenna beam to within +/- 0.1$DEG with a knowledge of +/- 0.05$DEG. The paper describes the techniques used to measure the performance of the AMSU-B antenna system and the results obtained. Using the techniques described, the antenna patterns of AMSU-B were measured over a dynamic range greater than 76 dB. Beam pointing was measured to within +/- 0.02$DEG.