Abstract: Regarding the architecture of a large-scale software as a directional network graph, where the class in the network and the relation between classes are thus referred to as a node and an edge respectively, a software network graph is given as an example. Then, a concept named betweenness is defined and calculated correspondingly. The BTS algorithm is proposed for the betweenness of network nodes and used to solve the betweenness of ten softwares with the computing time given. The results show that the magnitude of betweenness reflects the importance of a node in network and provides a guidance for developing softwares. It is found there is an increasing correlation between the betweenness and the product of accessibilites(degin × degout), and the correlation is fitted with the least square method in four softwares, thus giving the slope of the fitted straight line.