Abstract: The leaf blight of Platycladus orientalis in the hilly mountains of Xuyi County, Jiangsu Province is caused by Chlaroscypha platycldus Dai sp. nov. , a new parasitic cup fungus. This fungus can infect both young and matural trees. The current leaves are infected in June and the symptom appears in late February of the next year with a incubation period about 250 days. The apothecia are produced on the dead leaves of Platycladus arientalis in June, cup-shaped olive green with 0. 21 to 0. 46 mm in diameter. The asci are cylinder-shaped, no blue reaction with Metxer’ s reagent, eight spores within each ascus measuring 80-129×10-15 μm. Ascospores are oval, hyaline, and unicelular 15-18μm long and 8-15μm wide. The mycelium on PDA medium shows white to purple-red, produces olivecolour grains, but difficultly forms ascospores. The optimum temperature for ascospore germination is 25℃ to 30℃ and for mycelium growth is 15℃ to 25℃. The optimum pH for mycelium growth is 4. 5 to 6. 5.