Thelidiwn radiatum Harada sp. nov. is described on the basis of a specimen growing on sandstone rock in non-aquatic habitat in a laurel-leaved forest in Chiba- ken, central Japan. It is characterized by a grayish green, smooth, continuous epilithic thallus, exposed small perilhecia (0.2-0.3 mm diam.) with radiate cracks around prominently depressed ostioles, very dark brown involucrellum appressed to hyaline exciple, and medium-sized 1-septate spores (22-27 x 8-11 irn)