To deepen our understanding of those aspects of problems that cause the most difficulty for solvers, this study integrated eye-tracking with handwriting devices to investigate problem solvers' online processes while solving geometry problems. We are interested in whether the difference between successful and unsuccessful solvers can be identified by employing eye-tracking and handwriting. Sixty-two high school students were required to complete a series of geometry problems using pen tablets. Responses, including eye movement measures, wrote/drew trace, perceived cognitive load and questionnaires concerning the source of difficulties, were collected. The results suggested that the technique could enhance methods to diagnose difficulties by differentiating between successful and unsuccessful solvers. We considered mental rotation could be a primary obstacle in the integrating stage of diagram comprehension. The technique can be extensively applied in various instructional scenarios. Educational implications for problem solving are discussed.