A disease on mulberries with the typical symptoms of mosaic and dwarf leaves was found in middle areas of China in 1980s. Presently, this disease became serious and spread out. Based on previous finding, we detected a viroid-like small molecular RNA in diseased mulberries tissues. In this paper, we further identified the pathogen of mulberry mosaic dwarf disease (MMDD) according to the Koch's postulates and reported the diagnostic method of the pathogen by using PCR with two sets of specific primers. The result might be helpful to control the disease extension.