We studied the breeding ecology of Black-crowned Night Herons (Nycticorax nycticorax) in three mixed species heronries in Pyeongtaek, Gyeonggi-Do, Korea during 2000 and 2001. Population size and breeding species changed between 2000 and 2001. Mean clutch size, hatching success and fledging success was not significantly different between years. Fledging success varied significantly in 2000 but not in 2001 with regard to number of hatchlings per nest. Mass, culmen-length, and wing-length growth rates of Black-crowned Night Heron chicks did not differ between years. With regard to hatching order in 2000 and in 2001, only tarsus-length growth rates differed between years. Clutch size, reproductive success, and growth rate of Black-crowned Night Herons did not differ between years.