Background: Electroanatomical mapping allows a three dimensional localization of focal atrial tachycardia(FAT). However, reannotation of electrogram by operator is essential for localization of the focal origin and successful ablation. We evaluated the efficacy of the bipolar and unipolar electrogram as a guide to localization and ablation. Methods: Six patients (4 males, 47짹16.9 years) who underwent radiofrequency ablation guided by electroanatomical mapping for focal atrial tachycardia were enrolled. The time difference between surface P wave and the onset of bipolar(B onset) and unipolar electrogram(U onset) and steepest slope of unipolar electrogram(U slop) was measured. The time difference between the B onset and unipolar parameters also were measured. The amplitude of R wave of unipolar electrogram(R) was measured. All measured data were compared successful(6 sites) with unsuccessful site(25 sites). Results: Two patients had sinoatrial reentrant tachycardia, two had atrial tachycardia originating from the right superior pulmonic vein, one had from tricuspid annulus and one had from left atrial posterior wall. The onset time of both electrogram and U slop compared with P wave were not different between two groups. And the time difference between B onset and unipolar parameters were not different also. Only the amplitude of R wave of unipolar was significantly lower in successful site(P < 0.05). Detailed results are in the table. Conclusion: Electroanatomical map of FAT by annotation of earliest electrogram and following QS pattern of unipolar electrogram provides an efficacious guide to successful ablation.