Wulff’s relationship was proposed almost hundred years ago and this relationship is well accepted. This relationship was theoretically established, but no experimental evidence for this theory was demonstrated, because measurement of specific surface free energy of crystal surface was believed to be difficult. We have tried to measure the specific surface free energy of chlorapatite single crystal using contact angles of liquids and compared with its morphology. In that system, we could conform Wulff’s relationship. This time we adopt this technique for ruby single crystal in order to prove the generality of our technique to determine the specific surface free energy using contact angle of liquid. The contact angles of water and formamide droplets on ruby single crystals were determined at room temperature by sessile drop method. Although the contact angles have wide distributions, the average of the contact angles was determined after more than hundred repetition of measurement. The specific surface free energy of each face of the ruby crystal was calculated by Fowkes approximation and Wu’s mean equations. Wulff’s point was postulated at the center of the crystal and the length of the normal line to each face of the crystal from Wulff’s point was geometrically calculated and compared with the specific surface free energy. The ratio of specific surface free energy of the face and the normal length from center to the face of crystal was almost constant, which satisfies Wulff’s relationship. We are going to discuss about the physical meaning of Wulff’s constant from experimental viewpoint.

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