Reading ability is a fundamental and essential ability to lead everyday life. There have been attempts to diagnose and effectively guide learners in their reading ability. However, this was mainly centered on indirect and multiple-choice assessments. Therefore, this study focused on the eye tracker as a tool to supplement the limitations of indirect assessment and the shortcomings of the multiple choice question, and developed a reading ability diagnostic assessment using the eye tracker. For the reading ability diagnostic assessment using eye tracker, a reading ability factors was extracted and developed for each grade level. The assessment questions were revised and statistically verified through multiple test. In addition, based on reading ability, the students were sampled and applied reading assessment using eye tracker. These results were used to derive a reading ability diagnosis indicators. Reading diagnostic instrument that uses eye tracker is able to diagnose reading process. This provides an effective guidance by providing more accurate feedback to improve reading ability.