The medium of instruction (MOI) and its impact on improving the language proficiency of students has become a debated issue worldwide. The policy-makers of post-colonial Hong Kong believed that the general Chinese competency, Chinese writing and Putonghua proficiency of students would be improved through the use of a Putonghua-medium-of-instruction (PMI) rather than the Cantonese medium, the mother tongue of students and teachers. Therefore, using Putonghua to teach the Chinese language has been promulgated as a long-term goal by the Curriculum Development Council. Early studies have placed an emphasis on using standardized tests to assess students' learning outcomes with the use of PMI, but unfortunately the findings have been inconclusive. Despite teachers' involvement in implementing the MOI policy, there is a dearth of studies examining the perspective of teachers regarding the language learning of students under PMI. This paper aims to explore this important issue in greater depth through a case study of a high school, which has implemented PMI to teach the Chinese language for 10 years. The findings indicate that there are mismatches between the stated educational policy intentions and the language learning of students in various aspects. Further studies and follow-up measures on the language policy and education are suggested. (Contains 2 tables and 6 notes.)