Objectives: The variation of the substrates of verapamil-sensitive idiopathic left ventricular tachycardia (ILVT) was not understood. The purpose of this study was to investigate the variation of electroanatomic substrate [ slow conduction zone (SCZ) and left ventricular conduction system (LVCS)] in ILVT and control individuals and markers of successful ablation. Methods: Electroanatomical mapping was performed during sinus rhythm in 20 ILVT patients and 26 control individuals with paroxysmal supraventricular tachycardia. LVCS and SCZ were tagged in geometry and the anatomic aspects were investigated. Results: According to the distribution of Purkinje potential, LVCS was distinguished into three types: left bundle branch (LBB) was divided into two discrete fascicles without interconnections; divided into three separate fascicles; and fanlike structure distribution over septum broadly. The length of LBB and its fascicles in patients with ILVT were slightly longer than those of controls (P>0.05). In the ILVT group, the SCZ was located at the inferoposterior septum in 17, inferior apical septum in one and two SCZs were located at the posterior and mid-septal in the other two patients, which were greater in size and longer in length than those of six controls (P<0.05). At the crossover junction area with diastolic potential and Purkinje potential, with the size of 1.5 +/- 0.4 cm(2), concealed entertainment and ablation were obtained successfully in all patients with ILVT. Conclusion: The anatomy of the LVCS and SCZ is highly variable in patients with ILVT, and the crossover junction area with diastolic potential and Purkinje potential might be a marker of ablation.