MOI for sure affects the effectiveness of teaching, schooling and even language landscape of society. Under certain language policies or social language environments, selection of MOI is a metaphor of politics and resources. HK is a special place with a history of British colony over 150 years. After return of sovereignty to China in 1997, language landscape of HK does not change a lot. According to government statistical figures, Cantonese which HK people consider as their mother tongue is still a dominant language in daily life and local context (96.5% Proportion of the population able to speak Cantonese in 2006). This article is going to study how Putonghua becomes a core subject and Mol of Chinese Language subject in primary and secondary school curriculums from perspectives of policy making and responses of stake holders. According to scholars' research, by 2011, 46% of schools will adopt Putonghua as Mol for Chinese Language subject. Is enhancing Chinese standard of students the sole rationale behind this move? Can Putonghua, L2 or L3 of many Hong Kong students, enhance L1 Chinese standard of Cantonese speaking students? This is an interesting research question as Singapore is investigating how to use English, official L1 in Singapore, as Mol to teaching Chinese. As Putonghua has not developed from L2 or L3 to L1 in Hong Kong, is teaching Chinese Language subject in PMI a correct move right now? The findings of the paper can help us address the nature of L1 learning in multilingual and cultural societies such as Hong Kong and Singapore.