The purpose of this study is to use formal and informal diagnosis instruments to explore an EFL student's reading problems, and then apply a tutor's diagnostic instruction to increase his reading ability. The tests in this study consisted of Informal Reading Inventory, the Standford Diagnostic Reading Test, the Dolch Sight Word List Test, and the repeated reading. The objectives of diagnostic tutoring included increasing sight vocabulary, reading rate and fluency, word decoding skills and connecting the process of reading and writing. Through three-month tutoring, the subject improved two grade levels in both oral and listening comprehension and one grade level in word recognition. The present research suggests that diagnostic reading test and diagnostic tutoring can be applied to help EFL students deal with reading problems and increase their reading capability.