Satit Bilingual School of Rangsit University provides bilingual education to students by implementing most of its courses in English except for Thai and some parts of social studies that are directly related to Thailand. Consequently, there are certain doubts that the proportion of English versus Thai as medium of teaching currently in place (80:20) would cost students the loss of their mother language especially for younger students (primary 1-3) or the first class band.

From such doubts, The Faculty of Education, Rangsit University realizes the importance to study such an issue in a more detailed manner in order to assist the school’s management’s decisions on directions to improve Thai teaching effectiveness.

The research is conducted in order to analyze the students’ skills in mastering Thai language as well as skills in Thai sentence construction through essay writing.

124 students from primary 1-3 of Satit Bilngual School of Rangsit University were included in the study.

Subjects were asked to construct a sentence and communicate their ideas in a paragraph on the topic “My dream future” in 50 minutes and their short essays were analyzed by researchers from Faculty of Education, Rangsit University.

The result has shown great progress in subjects of different classes in mastering their Thai in subjects of different classes. Students from Primary 3 could complete three paragraphs essay within the given timeframe by structuring their essays into three parts namely introduction to the topic, elaboration of ideas and topics, and conclusion. The finding also suggests that students from Primary 1 and 2 could well master Thai by constructing sentences with the conjunction “because”. Students from Primary 3 could well master their Thai by constructing complex and compound sentences with the conjunction “because” together with other conjunctions including “as”, “if”, “even though”, “but”, “however”.

The research result indicated that students in a bilingual education system can master their mother language. Therefore, the immersion proportion given by the school can be argued to be an appropriate one.

The research also indicated that students make progress in communicating their ideas through essay writing by giving preludes to their ideas, elaboration and justification, and conclusion of their ideas. Researchers suggested the school’s management should improve their Thai instruction by asking students to construct more complex sentences by using more complex conjunctions for students from Primary 1-2. As for students from Primary 3, teachers can improve their writing and communication skills by asking them to write 5 paragraph essays.

The first paragraph of the essay should be the introductory part of the idea, paragraphs 2 to 4 should be the elaboration, illustration, and justification of ideas while paragraph 5 would be the conclusion. The research result can be utilized in the implementation level in order to improve SBS’s students’ Thai proficiency.