The mitral isthmus conduction block was inadvertently achieved during radiofrequency catheter ablation for a left lateral concealed accessory pathway (AP). we reported that the mitral isthmus incomplete conduction block accompanied by alternating atrioventricular conduction interval led to tachycardia cycle length variability during left lateral AP ablation. This rare electrophysiological phenomenon should be acknowledged to avoid radiofrequency applications in inappropriate regions.