A viroid-like disease causing mosaic leaves and dwarfism was found on mulberry plants in Zhejiang, China. Grafting of stems from infected plants onto healthy plants resulted in the same symptoms on the healthy plants. A circular small RNA (Mmd-v RNA1) was isolated from the infected plant leaves and caused identical symptoms after more than two years. Nucleotide sequencing indicated that the Mmd-v RNA1 contains 356 nt (GenBank accession no. NC_011362) without viroid characteristic regions, only sharing 30 nt sequence identity with that of the Cherry small circular viroid-like RNA 1 (isolate cscRNA1.150, GenBank accession no. DQ357628). This description is the first of viroid-like RNA infection of mulberry trees.