Abstract: The X-ray transmission property of mono-capillaries is the basis for designing X-ray lens. Based on the theory of X-ray transmission through capillaries, a detailed Monte-Carlo method is suggested in this paper. A corresponding computer program is worked out. For a given model, the transmission efficiencies of mono-capillaries versus diameter, incident focus and curvature radius are calculated. The transmission efficiencies of straight capillary reduce as capillary diameter increases. The transmission efficiencies of curved capillary increase as capillary curvature increase. The simulative results agree well with the corresponding experimental results．