Background: In the era of education reform when new teaching approaches are being advocated, it is important to investigate how different classroom discourse patterns are related to different educational outcomes. Aim: The purpose of the study was to analyse the whole classroom discourse of two teacher interns' writing classes. It was conducted against a background of education reform in which teachers were being encouraged to make major changes in their teaching approaches. We investigated what approaches were adopted and what discourse patterns were related to better educational outcomes. Sample: The participants were two teacher interns and their 61 students in Hong Kong. The students (35 boys, 26 girls) were seventh graders from two classes in the same secondary school. Method: An instruction unit of each teacher intern on expository writing (120 min) was video-recorded and analysed. Discourse analysis of the whole classroom interaction was supplemented by the students' perceptions and independent observers' evaluations of the instruction. Results: The results indicated that both teacher interns adopted a traditional direct instruction although student-centred approaches were being recommended in the education reform. The effectiveness of the instruction was associated with certain features of the classroom discourse patterns. Better educational outcomes were associated positively with utterances of high cognitive demand but negatively with utterances related to discipline. Students tended to have better performances in writing when they and the independent observers perceived that the teachers used more motivating strategies. Conclusions: The effectiveness of instruction is a complicated phenomenon that involves a myriad of interrelated factors. Teacher- or student-centred approaches are only part of these diverse factors.