The Ministry of Education has organized the teacher training--Boot Camp--to improve teachers' English proficiency and teaching methodology. Therefore, this study was conducted to investigate the extent of primary school English teachers' application of knowledge/skills obtained from Boot Camp to their classroom teaching practices and factors that hinder them from such application. Participants of this study were 100 primary school English teachers who attended Boot Camp, Batch 1-19, at Nakhon Si Thammarat Rajabhat University Center. A mixed method approach, combining a questionnaire and interviews, was used to gather the quantitative and qualitative data. The results showed that the teachers often applied the obtained knowledge/skills from Boot Camp to their classroom and factors that hindered them from such application are related to teachers, students, and school context. The results of this study provide implications for the design of the teacher training which promotes effective application of the knowledge/skills from the training.