Rod drop accidents (RDAs) were calculated for a typical 1100-MW(electric) boiling water reactor (BWR) using the three-dimensional core dynamics simulation code ARIES. Calculated cases are for cold start-up and hot standby cores. In both cold start-up and hot standby core RDAs, the moderator density reactivity feedback has an important effect on suppressing fuel enthalpy increase. Hot standby core RDAs, in particular, show remarkable effects of the moderator density reactivity feedback on reducing the power peak and succeeding fuel enthalpy rise. Sensitivity analyses of the effects of initial power level, core inlet subcooling, rod drop speed, dropping rod worth, etc., have been carried out under both cold start-up and hot standby core conditions for a typical 1100-MW(electric) BWR. In the hot standby core RDAs, the parameters affecting neutronic conditions (such as Doppler feedback) and moderator density proved to be very sensitive. In the cold start-up core RDAs, the parameters affecting moderator density are not so sensitive, but the parameters affecting Doppler feedback or neutronic conditions proved to be more sensitive than in the hot standby core RDAs.