Attention to teachers' Subject Matter Knowledge (SMK) as an assurance of quality education is a world-wide trend. However, the current legislation regarding the minimum SMK of teachers in Hong Kong (HK) is rather loose and research into analyzing the SMK of HK mathematics teachers is virtually absent. This paper describes a small scale research on probing into the SMK of HK primary school mathematics teachers. To set the scene for the research, a brief review was made on the 'knowledge base' of teachers and the concern about the SMK of HK teachers. Regarding data collection, a mathematics subject audit instrument used by researchers in England was adapted for an initial exploration into HK teachers' subject domain. Audit data were supplemented by qualitative data collected through interviews of participants. The collected data were analyzed and compared with the results of a mathematics subject audit at a teacher training institution in England. The SMK of an opportunity sample of HK primary school mathematics teachers was found to be relatively shallow, notwithstanding the outstanding mathematics attainment of HK pupils in recent international comparative studies like PISA and TIMSS.