The sinus node (SAN) is demonstrated to play a role in the genesis and the perpetuation of atrial fibrillation (AF). The sinus activation during AF is difficult to analyze with conventional electrogram recordings. The purpose of this study was to investigate the atrial substrate characteristics near the sinus node (SAN) during sinus rhythm (SR), ongoing AF and before AF termination according to the frequency analysis compared to the rest of the right atrium (RRA). The present study included 6 patients with paroxysmal AF underwent noncontact mapping for the right atrium (RA). After subtraction of QRS-T complex, regional frequency distribution was obtained from the signals of 256-equally distribution mapping site for three situations. The dominant frequency (DF), power amplitude (MP), and organization index(OI) of DF between SAN and RRA during AF were similar (p>0.1). However, the DF of SAN area became lower (p<0.001), the power amplitude of local spectra (p<0.005) was higher, and more organization (P<0.001) compared to RRA before termination of AF. In conclusion, atrial substrate around the SAN is characterized by slower atrial activity compared to the rest of the right atrium during termination of AF. The frequency analysis showed that the DF around SAN area during AF termination was similar to during SR, suggesting that the sinus automaticity was preserved with local entrance block from the rest of the atrium.