Objective : Min fire(民火) is mentioned as a kind of fire that exists in human body, which rarely appears in traditional medical books. It was mainly mentioned as a fire in urinary bladder, and was referred to have some function in the process of qi transformation(氣化) in urinary bladder. As the concept of min fire was originated from the Taoist internal alchemy, this study will make a comparative research between the examples of min fire in medical theory and Taoist internal alchemy, and will find out the conceptual significance of min fire in medical theory. Method : Among traditional medical books, min fire appears in such books as Yixuerumen(醫學入門), Youyoujicheng(幼幼集成), Fengshijinnangmilu(馮氏錦囊秘錄), and also in some books of Taoist internal alchemy. Though rather brief and informal, they show us some information about the performance of min fire in human body. So based on the information about min fire that was acquired from those books, this study will apply the concept of min fire to the traditional dispute on the problem of qi transformation in urinary bladder. Result & Conclusion : The concept of min fire that is related to the problem of qi transformation in urinary bladder, shows well that qi transformation in urinary bladder produces not only urine but also the ascent of shuijing(水精). And it also shows well that min fire acts as a power for the qi transformation in urinary bladder, which would perform well in the cooperation with sovereign fire(君火). This can be presented as a theoretical basis that the power for the qi transformation in urinary bladder is basically prepared in urinary bladder itself.