This study primarily investigated the validity and reliability of the writing assessments and their backwash effects on the undergraduates of Institute of International Studies, Ramkhamhaeng University (IIS-RU). The English-major students had academic writing skills problem, especially among the non-native English speakers, whose writing ability was critical to their academic achievement as they were required to produce many academic writing tasks. There were some native English speaking students who were also unable to write essays or compose properly. The IIS professors motivated their students to develop their writing skills by using writing tests (e.g., essay exam and writing prompt test) as the instruments to measure students' writing competence, ability, and knowledge in curriculum.