Tertiary collateral vessel（ Sun Luo） is a component of meridian and collateral system. It belongs to vessel（ Mai） in the five body constituents and is also a branch of collateral vessel. This article discusses the properties of tertiary collateral vessel after reviewing chapters and sections related to tertiary collateral vessel（ Sun Luo） and tertiary vessel（ Sun Mai） in Huangdi’s Internal Classic（ Huang Di Nei Jing）. It is found that tertiary collateral vessel is located at the level of flesh（ musculature） in five body constituents. A large number of tertiary collateral vessels form a wide range of interconnection in flesh（ musculature）

integrate Qi with blood

and flow to other structure

with particularly close relationship with Xigu and skin. Tertiary collateral vessel takes part in the circulation of Qi and blood

receives the infusion of nutrient Qi in middle energizer（ jiao）

and is a key part of meridian and collateral system for receiving the essence of water and food. The initial findings of the research reveals the complexity of tertiary collateral vessel as well as its distinctiveness in the Qi and blood circulation in human body. It has potential practical value in the diagnosis and treatment of related diseases

as well as in the research of chronomedicine. It is also significant for research on hemorheology of meridian and collateral system through further discussion on the characteristics of the structure and flow of tertiary collateral vessel.