Student learning in simple electric circuits has been extensively studied, which has revealed a large number of persistent misunderstandings. This study applies the conceptual framework model to investigate student difficulties in the knowledge integration perspective. The results are used to guide the design of a concept test that targets the different stages of knowledge integration in student learning of electric circuits. Specifically, two areas of research have been conducted. First, based on content analysis by experts and a review of the literature on students' conceptual understandings, a conceptual framework for electric circuits is developed. The conceptual framework model is then applied to guide the development of a multiple-choice concept test for assessment of knowledge integration in learning electric circuits. Both qualitative and quantitative data were collected from high school students who had completed the learning of electric circuits. The results confirmed that the conceptual framework model can effectively represent the knowledge structures of students at different levels of knowledge integration. In addition, the assessment outcomes also reveal that the concept test is effective in identifying unique features of knowledge integration, including context dependence and fragmentation of knowledge components, memorization-based problem solving, difficulty in transfer to novel contexts, and lack of meaningful connections between microscopic and macroscopic models of electric current. The assessment outcomes can also provide practical information for instruction to promote knowledge integration in learning electric circuits.