The purpose of this study is to prove the effectiveness of written expression program. To achieve it, We analyzed the effects of self-monitoring strategy for spelling and written expression with Tier 3 intervention. In this research, RTI Tier 3 intervention was conducted by utilizing self-monitoring strategy to elementary school students with low writing achievement. The study was used multiple baseline design across participants. In this study words with phonological changes occurring in the Korean language were taught. Also in this process students were got intervention directly by adapting RTI Tire 3. The results of this study are as follows. First, Tier 3 intervention based on self-monitoring program and phonologically changing rules was adapted and it was effective on general evaluation and formative evaluation. Second, Tier 3 intervention based on self-monitoring program enhanced written expression skills in fluency. However, it did not show meaningful effects on quality. This research analyzed not only the spelling skills but also writing fluency skills as part of writing expression with students’ self-monitoring strategy. As a result, by using this program to students who suffer from spelling, it could be effective program.