This study investigates the effects of seismic damages and retrofitting on dynamic characteristics of substandard reinforced concrete buildings. Two identical 3-story buildings were constructed, one of which was retrofitted through application of carbon fiber-reinforced polymer jacketing on its columns. Ambient vibration tests were conducted at undamaged states, and after formation of damages of different extents. Test results showed that even limited reductions in vibration frequencies could indicate serious problems about structural safety. Although the vibration frequencies of both buildings were observed to be decreased with the increasing level of damage, decrease of those of the retrofitted building was lower.