The purpose of this study is to identify the entry conditions that could form a co-evolutionary relationship between a sewing contractor company and a fashion brand, and reveal the outcome of such a relationship. In the Republic of Korea, many fashion brands and sewing companies coexist, and the situation in the fashion industry changes with the rapid development of the country, so it is a suitable environment for investigating their co-evolution strategies. A qualitative research methodology was used to examine the co-evolution process of the fashion brand and the sewing industry. In addition, an objective evaluation was conducted by using interviews with sewing contractor companies and fashion brands that have business relationships with the sewing contractor companies. The results indicated that sewing contractor companies evolve into either mass production or small production systems. In the co-evolutionary process, relational resources and business-to-business transaction suitability are the main entry conditions for sewing contractor companies to coevolve with fashion brands. Knowledge exchange, experience optimization, off-season ordering, and investment in trading companies were observed among sewing contractor companies that formed a co-evolutionary relationship with their fashion brand partners. This study identified and conceptualized factors that played a major role in the process of co-evolution and proved suitable for each production system of sewing contractor companies.