Abstract The peak time for the security investment trust company number is in 2004 and 2005, with a total of about 45 companies at that time. However, the number of companies are decreasing. Presently, only 39 in the security investment trust industry are continuing. Observing the current situation of the security investment trust industry, it could be found that companies with large asset size are permanently staying large. In addition, companies in the behind sector are facing difficulties with management, abatement of cash flow and deterioration of constitution that all cause a vicious cycle. This research is studied by quality methodology, through initial data from in-depth interviews and secondary data of the industry. The analyses of Porter¡¦s Five Forces and external industry environment are used in order to understand the competition of the security investment trust industry and its profitability in a long-term. By in-depth interviews, the analyses of competitor benchmark and core competences are diagnosed so as to find out reasons why these behind sector companies in the past can reach the top 10 in the asset scale now. Eventually, core- competences and direction of constitution for medium size firms can be built via case studies. The result of this study discovers that core competences of a company are formed by its own advantages, and core competences of an enterprise are built via an enterprise¡¦s strategic resources. The leverage and consolidation for resources can help to create sustainably competitive advantages. By attestation and conclusion, these top 10 companies are able to properly implement enterprises¡¦ resources for product innovation, trade record and channels. They all have clear strategic positions and distinctive business models.