High‐TcYBa2Cu3O7rf superconducting quantum interference devices made by oxygen‐ion modification have been produced and used successfully since 1990, but the mechanism of their operation remains still unclear. Reported is a computer modeling of the ion modification using theTRIMprogram. It appears that the depression of the transition temperature is responsible for the operation of these bridges as weak links. At 77 K and modeling using our experimental parameters, only the base of the YBa2Cu3O7film about 50 nm thick near the substrate is superconducting, which acts as a weak link probably because of the temperature‐dependent coherence length near the reduced transition temperatureT′cof the modified material. ©1995 American Institute of Physics.