The present invention relates to a composition for preventing and treating diabetes mellitus that includes a shikonin compound as an active ingredient and to a use thereof. More particularly, the present invention relates to a composition for preventing and treating diabetes mellitus that includes isobutyryl shikonin, .beta.,.beta.-dimethylacryl shikonin, isovaleryl shikonin, or .alpha.-methyl-n-butyryl shikonin as an active ingredient and to a use thereof. The composition of the present invention exhibits antidiabetic effects by blocking the K.sub.ATP ion channels (ATP-sensitive potassium ion channels) of beta (.beta.) cells in the pancreas and increasing the concentration of calcium ions to promote the secretion of insulin, and thus can be used as a medicinal pharmaceutical, food additive, or health functional food for preventing and treating diabetes mellitus.