Letterer- Siwe disease（LSD） is a kind of a histiocytosis, rarely seen in clinic. However, its prognosis is poor. A case of LSD which complaints of the oral ulcer is reported in this article. The patient of the case was a boy, came to hospital on July 5, 2003 because of oral ulcers, teeth loosing and falling off for a month, and pus running over from both antra auris for two weeks. No familial history of genetic diseases and special infections were found. The case consisted of the following features after exam: Erythematous papules and petechia dotted in the skin, primarily on scalp and trunk，two ears suffering otitis media, oral proliferative ulcers appearing in palate and lower gingival accompanying with the lower teeth loosing in the lesion area, in the meantime, the destruction of madibula on the left side also found out by X- ray, hepatic dysfunction and poor cellular immunity coming along with pertinacious candidosis. The pathologic exam showed histioid cell infiltration, and the immunohistochemical exam showed CDla（+）, S- 100（+）. Diagnosis defined it LSD. According to the case′s character and literature review, the clinical features, diagnosis and therapy of LSD were discussed.