Abstract: A new type of dolomite,which is called characteristic dolomite in this paper owing to its abnormal morphologies of "neelde", "tabular-prismatic", "wheat" or "dendritis" forms, occurs on a wide range of scales in the fourth member of the Trassic Jialinjiang Formation on the Upper Yangtze Platform. Unlike the normal sedimentary dolomite,the dolomite may be formed by pseudomorphous replacement of fibrous and dendritic calcite as a result of the similarity both in morphologies and intergrowth patterns. The occurrence of the characteristic dolomite indicates that sedimentary and/or diagenetic environments are characterized by meteoric vadose,medium fluids supersaturated with respect to calcite,and plentiful micro-organisms,which created some conditions suitable for the growth of characteristic calcite and dolomite.