The meridian theory(經絡學說) is one of the important Korean medical theories distinguishing it from western medicine. The meridian theory(經絡學說) can be divided broadly into meridians(經脈) and collaterals(絡脈). The studies on meridians(經脈) have been relatively advanced, but the studies on the collaterals(絡脈) has not been enough progressed so far. Fifteen main collaterals(十五絡脈) are the biggest and most important part of collaterals(絡脈). Unlike other collateral(絡脈), fifteen main collaterals(十五絡脈) have certain routes and their own collateral acupoints(絡穴). So we studied the structure of collateral(絡脈) mainly on fifteen main collaterals(十五絡脈). In addition, we searched the Asian symbolic-mathematical(象數學的) meaning of the fifteen main collaterals(十五絡脈) and newly described them. As a result, the concept of collaterals(絡脈) has been created by the accumulation of clinical experience later than that of meridians(經脈), and it has been formed while the meridian theory(經絡學說) were developed. The meaning of ‘fifteen’ in fifteen main collaterals(十五絡脈) could be analysed in three ways based on the result of symbolic-mathematical(象數學的) study. According to those results, we could find that the structure of fifteen main collaterals(十五絡脈) in 『Internal Classic(內經)』 was accurate.