This paper is concerned with the damping ratios of two methods, which are frequency domain and time domain approach. Ambient vibrations and synchronized human excitation test were conducted to three reinforced concrete buildings ranging from eleven to nineteen stories. The performance of the half power bandwidth method was investigated using three kinds of sample size, 1024, 2048, and 4096. The damping ratio by synchronized human excitation ranges from 1.05% to 1.22% in the long direction and from 1.16% to 1.50% in short direction. Damping by half power bandwidth method is slightly more overestimated than the synchronized human excitation due to insufficient record length. Damping evaluation by half power bandwidth method was found to be enhanced by using the narrower bandwidth with long recorded data.