Posteroseptal accessory pathways located in the CS diverticulum are rare and unique forms of accessory pathways which are difficult to precisely locate and ablate. The ablation of this type of accessory pathway could also be associated with collateral injury to the neighbouring coronary arteries. We present a case of Wolff--Parkinson--White syndrome due to a CS diverticulum accessory pathway with previous failed ablation attempt who was safely and successfully ablated. We demonstrated that using the advanced 3-D cardiac mapping system could guide clinicians to precisely locate and safely ablate these difficult pathways.