"Switching the Infant Crown Prince to a Leopard Cat" (貍貓換太子) is one of the well-known stories of Bao-Kung's inquisitions (包公案). The story began with the emperor, Sung-Zen-Chung (宋仁宗), who found his real mother, Queen Lee (李宸妃) after his step mother, Queen Dowager Liu, (劉太后) died. Within the efforts of many folk historians and writers, the "Switching the Infant Crown Prince to a Leopard Cat" gradually became a famous legend. The story of "Switching the Infant Crown Prince to a Leopard Cat", had been developed into many different forms of folk literatures. There had been many elements involved in transferring it from a simple story to a well-known legend. This paper aims to study from the base of ancient studies related to the story of "Switching the Infant Crown Prince to a Leopard Cat", and to discuss what has caused the formation of the story. It will surely help us to understand the development and the real meanings behind the story.