This study analyzed Taiwanese elementary school English teachers' perceptions of, designs on, and knowledge constructed about, differentiated instruction in content in an intensive summer course. Based on the data analysis of surveys, the document, videos, and an interview, the study had the following findings. First, before the intensive summer course, although they thought highly of differentiated instruction, they used the same textbook and did not implement differentiated instruction in their classroom practice. Their reluctance resulted from their lack of competence in differentiated instruction, time, and opportunities on collaborative planning. Secondly, throughout the intensive summer course, only three teachers differentiated their instruction in content by integration of jigsaw reading and supplementary materials. Finally, their knowledge constructed on differentiated instruction mainly came from the instructor of the intensive summer course and online resources. Three suggestions are provided to help elementary school English teachers effectively implement differentiated instruction in content in their classroom practice.