The aim of this study was to analyze factors influencing transfer of training in in-service training for vocational training instructors using Success Case Method (SCM). To achieve the purpose of the study, a survey was conducted to 458 trainees who completed the in-service training for vocational training instructors in the first half of 2021 to analyze the degree of training transfer. Then, a total of 20 trainees who showed a high level of transfer were finally selected, and in-depth interviews were conducted to identify the success factors for trainees with high level of training transfer. The results of the study show that learning and utilization ability, self-efficacy, self-reflection through training contents, motivation of transfer, and motivation of learning were derived as individual factors that facilitate transfer of training. In addition, the instructor’s training methods, the instructor’s expertise and training skills, and the job relevance of the training contents were found as the training design factors. Furthermore, organizational environmental factors include the support from affiliated organization and the support from colleagues. Finally, the networking with the trainees was found as another facilitating factor of training transfer. Based on the results of this study, implications for improving the transfer of in-service training for vocational training instructors were suggested.