Superconducting accelerating cavities made of Nb‐coated copper were produced. Niobium films of a thickness ranging from 1.4 to 4 &mgr;m were deposited onto the inside of 3‐GHz cavities and 500‐MHz frequency by bias diode sputtering. A maximum accelerating field of 8.6 MV m−1was reached without quench which is attributed to the large thermal conductivity of copper at liquid helium temperatures.