The purpose of this case study is to investigate whether a diagnostic reading test and diagnostic teaching can help students who encounter reading difficulty to solve reading problems and promote reading competence and potential. The tests in this study consisted of informal reading inventory, the Stanford Diagnostic Reading Test, the El Paso Phonics Survey, the Slosson Oral Reading Test, and the Repeated Reading. The objectives of diagnostic teaching included increasing sight vocabulary, improving the ability to decode vowel diphthongs, increasing his reading rate and fluency and connecting the process of reading and writing. Through three-month tutoring, the student improved five grade levels in both word recognition and oral comprehension. The research suggests that diagnostic reading test and diagnostic teaching can be applied to help ESL/EFLstudents overcome reading problems and improve their reading competence and potential.