The purpose of isolation condensers (ICs) in a boiling water reactor (BWR) is to passively control the reactor pressure by removing heat from the system during abnormal reactor operation. This type of control is expected to reduce the frequency of opening and closing of the safety relief valves (SRVs). In addition, the ICs are designed to remove the decay heat of the reactor system after reactor shutdown in case of a loss of coolant accident. Transient calculations of an anticipated transient without scram for a main steam isolation valve (MSIV) closure event were performed using the RAMONA-4B code with prescribed pressure set points for the SRVs and given time settings for the MSIV closure. It is demonstrated that the effect of the ICs is to reduce the rate of pressure rise and thereby decrease the cycling frequency of the SRVs. The onset of high-pressure coolant injection is delayed if an IC is functional. An analysis for natural circulation flow in a BWR indicates that the effect of an IC on the transient performance is similar to that for the forced circulation system. In this case, the MSIV closure results in a lower peak pressure because of the lower power level.