A natural circulation boiling water reactor (BWR) with a rated capacity of 600 MW(electric) has been conceptually designed for small- and medium-sized light water reactors. The components and systems in the reactor are simplified by eliminating pumped recirculation systems and pumped emergency core cooling systems. Consequently, the volume of the reactor building is ∼50% of that for current BWRs with the same rated capacity; the construction period is also shorter. Its thermal-hydraulic characteristics, critical power ratio (CPR) and flow stability at steady state, decrease in the minimum CPR (ΔMCPR) at transients, and the two-phase mixture level in the reactor pressure vessel (RPV) during accidents are investigated. The 8 × 8 fuel bundles with 3.1-m active lengths are used to achieve high seismic resistance and good thermal-hydraulic characteristics. Operation pressure of 7.0 MPa and volumetric power density of 34.2 kW/ℓ are determined from the CPR and flow stability limitations. The maximum ΔMCPR appears at load rejection transient and is <0.05. The CPR under normal operation is >1.3, which is a sufficient margin for the limitation value of 1.12. The two-phase mixture level in the RPV during an accident does not decrease to lower than the top of the core; the core uncovery and heatup of fuel cladding would not occur during any loss-of-coolant accident.