This study was designed to develop a scale to measure primary school English teachers’ professionalism based on affective factors. The Swailes' (2003) scale was used to best reflect the local context where teachers in primary schools teach English to young learners. Three surveys were sent to a total of 1,008 primary school English teachers. Three different focus groups were used to refine the survey items in the modification process from the first to the third survey. The data were analyzed using exploratory factor and confirmatory factor analysis. These results showed the validity of fifteen items for investigating primary English teachers’ professionalism: four items related to 'professional commitment', three related to 'professional autonomy', three items related to professional belief in public service, three items to profession as referent, and two items to professional regulation. It is hoped that the scale items developed in this study can be used to identify the standard of professionalism of primary English teachers and provide policy makers or school decision makers with data that can be used to enhance professionalism within the community of primary English teachers.