Ornithine transcarbamylase localized in the mitichondria is the second enzyme in the biosynthesis of arginine and urea biosynthesis. Defect of OTCase can cause urea cycle disease. OTCase is made in the cytosol as a monomer and translocated into mitochondria where they form a trimer. In this review, the catalytic mechanism, structure and the study OTCase were disscussed.