Background: Local electrograms recorded from the coronary sinus provide important information for the diagnosis of various arrhythmias and identification of target sites for ablation of left-sided accessory pathways. However the large anatomical discrepancy between coronary sinus (CS) and mitral annulus (MA) was sometimes observed. Objectives : The purpose of this study was to evaluate the anatomical relation between CS sinus and MA in fluoroscopy. Methods: This study population consisted of 20 patients who had 16 paroxysmal supraventricular tachycardias, 3 paroxysmal atrial fibrillations and 1 ventricular tachycardia. During executing ablation, left atriogram was performed using pig tail catheter via transseptal approach. The distances were measured in RAO 30째 projection during end of ventricular systole and diastole. Results: The distances between CS and MA were 4.1 짹 0.1, 5.0 짹 0.1, and 5.6 짹 0.1 mm in systole and 5.6 짹 0.1, 6.9 짹 0.1, and 6.8 짹 0.1 mm in diastole at proximal, middle, and distal CS respectively. The longest distance of them was 19.3 mm. The distance between CS and tip of ablation catheter at successful sites were 7.0 짹 0.0 mm in systole and 12.5 짹 0.1 mm in diastole. The distance between MA and tip of ablation catheter was 3.3 짹 0.0 in systole and 3.7 짹 0.0 mm in diastole. The ratio of atrial and ventricular amplitudes were 1.3 짹 0.0, 1.3 짹 0.0, and 1.3 짹 0.1 at proximal, middle, and distal CS and 1.1 짹 0.0 at ablation site. Conclusion: There are variable anatomical discrepancies between CS and MA. Therefore CS electrocardiogram can not exactly localize left sided bypass tract. The precise understanding of anatomical and electrophysiologic relation between CS and MA is needed for the diagnosis and treatment of arrhythmia.