The earliest paper currencies in existence in China were handcrafted during the Yuan dynasty. These currencies were scientifically excavated from different ruins or tombs, whereas scientific analyses of the papers are rare. This study used optical and scanning electron microscopy to examine the fibers collected in Yuan dynasty paper currencies in conjunction with the Herzberg staining method. Despite differences in circulation period, paper fibers in both Zhi Yuan Tong Xing Bao Chao (two Guan) and Zhong Tong Yuan Bao Jiao Chao (one Guan and 500 Wen, issued in Zhi Zheng period) were identified as similar papermaking materials, bast fibers of mulberry bark. The results indicate that mulberry bark, a durable papermaking material used since ancient times, was mainly utilized as a raw material in these Yuan dynasty paper currency. This fiber identification work solved the critical problem of papermaking material in the Yuan dynasty paper currency and provided important information for conserving these precious cultural relics.