As teaching English as a foreign language, it has become obvious that an average Thai is considered to have very low English proficiency. As a result, Thailand may find herself at a disadvantage because of inferior English skills towards globalization and forming of ASEAN community. Thus, the study is devoted to the exploration of English writing skill which sets out to obtain some concrete information on the students' problems in writing English supported by Systemic Functional Grammar. The result reveals that five problems of the Theme in terms of Theme selections and Thematic progression patterns were found: the problem of empty Theme, the problem of the brand new Theme, the problem of overuse of constant progression, the problem of empty Rheme, and the problem of confusing selection of textual Theme. It is hoped that the current study would be a contribution to the development of linguistic studies. This study will provide not only Thai EFL learners with a better understanding of problems in order to help them improve their writing performance, but will also provide Thai teachers with information that will help to develop English teaching materials and practical tools for Thai EFL learners, especially for teaching writing in EFL classroom.