Tongue diagnosis is a profound and special part of the whole Oriental Medicine. We examined the method, the principle and the meaning of tongue diagnosis according to a literature cited, considered a meaning of tongue diagnosis. As a result, we come to a conclusion like that. 1. Tongue is related with internal organs by meridian system, especially has a direct connection with heart and spleen. 2. The heart, a master of internal organs, has its specific opening in the tongue. The spleen, source of nutrients for growth and development, has its specific body opening in the mouth. So tongue reflects states of internal organs, Qi, blood, the constructive energy and the defensive energy. 3. When doing tongue diagnosis, we must pay attention to the position and the order of inspection of the tongue. We must diagnose by referring to a ray of light, diet, season, age, physical constitution, habit and taste, can make a accurate diagnosis. 4. We can classify constitutions, distinguish syndromes, suppose prognosis, make a prescription by using tongue diagnosis. 5. Reddened tip of the tongue represents flaring-up of the heart fire, and it means psychologic stress. Dental identations on the tongue edges represents deficiency of Qi of the spleen, and it means physiologic fatigue. 6. Through observing humidity of fur of the tongue, we can guess psychologic stress and physiologic fatigue. Through observing thickness of fur of the tongue, we can guess function of spleen and stomach.