To discuss the essence of six meridians,the key point is the comprehension of three yin and three yang.Three yin and three yang describes the six natural states of re-division and reunion of yin and yang.The opening,closing and pivot of three yin and three yang determine the property and characteristic of each Meridian.It is necessary to understand three yin and three yang by the theory of five movements and six qi as the comprehension of different states in space-time,azimuth and yin-yang qi.If we discuss six meridians from the view of the theory of five movements and six qi,most of the previous puzzles could be solved reasonably.The dynamic space-time variations among three yin and three yang is well-ordered.Syndrome-differentiation according to the theory of three yin and three yang could well reflex the dynamic space-time characteristics of the whole changes of internal and external environments when disease happens.The theory of six meridians must be mentioned as lecturing Treatise on Cold Pathogenic Diseases(Shanhan Lun),and the syndrome-differentiation of the six meridians can not be abolished.