The purpose of this research is to investigate the effectiveness of a two-week-intensive English learning program of Yokohama College of Commerce at its affiliated school in the US, at the University of Pittsburgh at Bradford. In 2019, one of the authors of this research led the group of students and observed the transition of the students not only in the aspect of language ability but also the motivational aspect during and after the program. All the students took All-in-English classes delivered by an American professor and participated in other activities both on-and-off campus. Students were satisfied with their L2 self-esteem and became more self-content while their motivation to study English considerably strengthened throughout the program. However, some students demonstrated little improvement in their test scores. The central research question is ‘what are the influential factors that improved their English’? There are students who improved their post-program English test scores and those who did not improve. To find out what hindered their English acquisition and what helped them, both quantitative and qualitative research was conducted. The authors conducted individual in-depth interviews as well as questionnaires after returning to Japan. Students also wrote feedback reports in the form of a post-program assignment, and they were analyzed using the text-mining method. Students also took English proficiency tests before and after the program, and their scores were compared and analyzed. The research results of this study will be used to improve the program next year.