The diffusion‐bonding method has been applied to the fabrication of aC‐band (6.5‐GHz) superconducting Nb cavity of TM010mode. The cavity consists of two parts which are machined from a reactor‐grade solid billet of Nb. The two parts are electropolished and joined in an ultrahigh‐vacuum induction furnace by applying mechanical pressure of about 1.7 kg/mm2. For the bonding, it takes about 10 h at 1900 °C in 10−7–10−8Torr. A tension test shows that the tensile strength of the joint is more than 24.4 kg/mm2. The cavity, which is vacuum annealed in the same furnace and lightly electropolished, shows an unloadedQas high as 1.1×109at 1.3 K in the rf field region of 0–7 G. The maximum surface field as high as 321 Oe and 22.7 MV/m is obtained. These results are compared with those obtained for cavities fabricated by the electron‐beam welding method.