Anticipated transient without scram (ATWS) has received much attention since the beginning of the last decade. It has been recognized as a dominant accident sequence for possible core melt and containment damage in a boiling water reactor (BWR) power plant. In the literature, a great deal of study has been reported on the investigation of BWR behavior during an ATWS, especially for BWR/2 and BWR/4. The objective of this study is to assess reactor behavior during a main steam isolation valve (MSIV) closure ATWS for the Kuosheng nuclear power plant of Taiwan Power Company, which has two units of BWR/6 with Mark-III containment. The analyses were performed using RETRAN-02/MOD3, which solves a one-dimensional homogeneous equilibrium model of two-phase mixture. The relative motion between two phases is treated by the dynamic slip model. Due to the unavailability of appropriate three-dimensional neutronic codes, such as SIMULATE-E, in this country, the option of point kinetics has been adopted for calculations. This approach is not expected to give the exact power shape variation during the transient, but is believed to be appropriate for providing the accurate overall power level, which is the major concern for core and containment integrity.