Since the 1960 s, domestic and foreign scholars have exp lained the intra - industry trade from theoretical and emp irical prospects and put forward their own different understandings of the concep t and measurement indicator of intra - industry trade. Those concepts are put forward on SITC or substitutability basis, in order to measure the level of intra - industry trade they also give a series of measuring indicators, each one of which is in accordance with the scholar’s own understanding of the concep t and his or her empirical research needs. Those indicators can fall into two categories: traditional static indicators and bilateral dynamic indicators, among which G - L index is used most frequently.