Mechanical properties of LiB₃O₅ single crystal plates with different orientation as well as of glass with the same composition have been investigated. The nano- (H) and microhardness (H_M), the reduced Young's modulus (E_r) and the crack behaviour of the samples were studied. Both hardness and Young's modulus of glass appeared smaller in comparison to corresponding single crystal data (H ∼ 1 - 8 GPa, H _M∼ 6 GPa, E_r ∼ 70 - 80 GPa for glass and H∼ 10 - 15 GPa, H_M∼ 6 -11 GPa, E_r ∼ 93 - 155 GPa for single crystal). H, E_r, and the plane of crack propagation proved orientation-dependent. Cracks in the glass sample were not observed up to 0.49 N microindentation load, whereas for the single crystal the cracks appeared already at 0.098N. In single crystals the observed cleavage planes {211} and/or {412} are oriented nearly parallel to planes of B-O rings.