The design of an electron nuclear double resonance (ENDOR) resonator suitable for single crystal studies is reported. An outline is given of the mechanical and electronic construction of the resonator. This paper will focus on the construction suitable for studies at 4.2 K, but the design can be easily adapted to a flow cryostat for variable temperature measurements. The rf‐coil efficiency has been measured and is high and fairly constant over the frequency range from 0.3 to 20 MHz. Measurements on a test compound, show that the baseline stability is good over this frequency range. Even for low abundant nuclei as13C, the signal to noise ratio is very good.