We report the observation of radio frequency‐superconducting quantum interference device (rf‐SQUID) effect due to natural grain boundary weak links in a bulk sample of a YNi2B2C superconductor at 4.2 K, confirming that the superconducting grains are coupled at the grain boundaries through Josephson effect. Periodic oscillations in voltage‐flux characteristic due to rf‐SQUID effect have been clearly observed at 4.2 K when the driving rf frequency was kept slightly away from the resonance frequency of the tank circuit. Phase reversal in the voltage‐flux characteristics on changing the rf frequency has been observed. Noise spectrum of the YNi2B2C bulk rf‐SQUID is also reported. ©1996 American Institute of Physics.