Abstract： Background The transseptal puncture technique has been widely used in therapeutic left atrium catheterization.But this technique may lead to some life-threatening complications.It can not be used widely because it is restricted by economy and deferring the transseptal puncture procedure.The aim of this study was to introduce a simple and safe transseptal puncture procedure.Methods The distal of coronary sinus (CS) electrode was positioned close to the lateral margin of heart,which was about at 3 O'clock at left anterior oblique (LAO) 30° referred to whole cardiac profile.It was then used as the marker for the level of fossa ovalis at posterior-anterior projection to guide the pull of transseptal needle.The midpoint between the distal CS and the posterior margin of heart at right anterior oblique (RAO) 45° view was considered as the location of fossa ovalis.Once the puncture was succeeded,the guidewire was introduced to the left superior pulmonary vein via puncture sheath after the needle was retrieved.The end of outer sheath was introduced into left atrium with the protection of guidewire.It was applied in 539 patients (316 male,223 female; (53±16) years old) who underwent catheter ablation of atrial fibrillation or left-sided atrioventricular accessory pathway.Results This transseptal approach reached 100.0％ success and was succeeded in 98.9％ with the first attempt.The first attempt puncture was aborted due to greater resistance to needle advancement or smaller needle curve in six patients.However,the second attempts were all succeeded after the needle curve was reshaped.There was no tamponade and embolism occurred.Conclusion The atrial septum puncture approach using the location of distal CS electrode as important marker and the guidewire for protection when sending outer sheath into left atrium is reliable and safe.