In this case study, the effects of Modified Cognitive Strategy Instruction (MCSI) on the story writing skills and procedural knowledge of writing process of a student with mild intellectual disability was investigated. The student is attending the 6th grade special education class. MCSI was designed in accordance with the instructional properties of Cognitive Strategy Instruction in Writing and Self-Regulation Strategy Development. In this study, the stories of student were assessed respect to the story elements, quality and length. Furthermore, the time spent by the student for writing was also calculated. In order to determine student's procedural knowledge of writing process, metacognitive interview was administered. In consequence of instruction, the student showed improvement in terms of all the study variables. In the study, the results were discussed and suggestions for further implementation and research were provided.