The rotational spectrum of the CoF radical in the X 3 Φ 4 state was observed by employing a source modulation microwave spectrometer. The CoF radical was generated in a free space cell by a dc glow discharge in CF 4 and He. The cobalt atoms were supplied by the sputtering reaction from cobalt powder placed over a lower surface of the cylindrical electrodes. The transitions with J=11–10 to 17–16 were measured in the region between 250 and 400 GHz. The effective rotational, centrifugal distortion, and hyperfine constants were obtained by a least-squares analysis.