香港學界對應否以普通話作為中文科教學語言（「普教中」）意見分歧，其中一個爭論就是「普教中」能否提升學生語文水平。過往的研究甚少深入「普教中」課堂探討教學過程與教學效果的關係，本文則詳析一節「普教中」課堂，並與一節以廣州話教授相同內容的課堂比較，探討課堂教學情況，及其與學生學習成果的關係。研究發現「普教中」班別受到語言轉譯問題而影響了教學成效。本文建議日後的研究應深入分析「普教中」課堂，多做粵普課堂教學的對照研究，探討可作改進之處，以及如何支援教師有效以普通話教授中文課。

There is a discrepancy in whether to use Putonghua as the medium of instruction in Chinese teaching (PMI) among scholars of the educa-tion circle in Hong Kong. One of the rationales points to the effective-ness of PMI on improving pupils' Chinese language level. In the past, there were few studies or the relationship between the teaching process and outcome in a PMI classroom. In this paper, a PMI lesson was ana-lyzed and compared with a lesson with the same content but was taught in Cantonese (CMI). The relationship between teachers' instruction and students' learning outcome was explored. Evidence showed that there was adverse effect on students' learning in PMI class due to the problem of language translation between Cantonese and Putonghua. It is therefore concluded that there should be an in-depth analysis of PMI lessons and comparative study between PMI and CMI lessons so to make further improvements and support teachers to teach in Putonghua more effectively.