BACKGROUND: Radiofrequency catheter ablation(RFCA) therapy was emerging treatment in the patient with highly symptomatic, drug refractory atrial fibrillation(AF). Main target of RFCA in AF is posterior wall of left atrium(LA) and pulmonary vein(PV). So the data for PV anatoomy is essential. But Racial difference maybe present in structure of left atrium. METHOD :We studied cardiac MRI(GE, signa CV/i 1.5 tesla) in 55 patient before RFCA for AF in St. Mary���s hospital. Left atrial AP diameter, width and hight were measured. PV was assessed in number, ostial size, angle of ostial plane, in axial and coronal plane. Angle of septum in axial and coronal plane also measured. RESULT: LA size is was 32짹6.7mm(AP), 67 짹 8.2mm(width), 54 짹 6.5mm(hight). Left common trunk was found in 4 patient(7.3%) and right common trunk in 1(1.8%). Extra os exeed 2 in either side was found in 1(1.8%) patient on left side and 3(5.5%) patient on right side. PV size was shown in table. Angle of interatrial septum is 36.8짹 5.9 째in axial plane and 35.0짹7.5째 in coronal plane. CONCLUSION: Measurement of left atrial structure was presented. Modification of routine RFCA technique may be necessary according to anatomical information of Korean population.