The quality of teacher-child discourse affects children's development and learning. Our study aims to explore teacher-child discourse during planned science activities (PSAs) in Vietnamese preschools. This research is essential to understanding the reasons behind Vietnamese children's low level of scientific skills and to exploring the dynamics of discourse in an Asian context. We conducted a case study to describe and understand the teacher-child discourse during PSAs. The study involved eight PSA sessions in four classes of three to four-year-old in Da Nang, Vietnam. We found that teachers tended to make statements more often than ask questions. When they did ask, they preferred using closed questions that did not require much of a response from children. Open questions were used sparingly, even though they are crucial for promoting children's cognitive thinking and literacy. Teachers also made use of rhetorical questions that did not require any answer. This has led to mostly non-interactive classes where students remain passive learners. To improve the quality of teacher-child discourse, we suggest providing teacher training on encouraging children to participate and asking more engaging questions.