The authors discuss methods to predict future earthquake damage and assess actual one to RC structures using numerical simulations and vibration measurement. First, suitability of the Applied Element Method (AEM) is examined as a tool for measuring the seismic performance of RC structures with and without retrofit. AEM can simulate the damage behavior of RC columns, jacketed RC columns and real railway viaducts. Next, a method to improve the accuracy of vibration diagnoses of earthquake-damaged RC structures is discussed using damage assessment criteria calculated by AEM. The changes to natural frequencies caused by damage to RC columns and an actual railway viaduct with steel jacket were also correctly estimated.