The purpose of the present study is to examine content knowledge (CK) and pedagogical content knowledge (PCK) of Greek teachers in number sense and specifically in mental calculations with rational numbers (fractions, decimals and percentages). Examined within the framework of CK were the type of strategies employed by teachers and the extent of the repertoire of these strategies, which provides an indication of their flexibility. Teachers' CK performance in mental calculations with rational numbers was compared with the extent of their strategic repertoire as well as with the PCK they employed when teaching mental calculations with rational numbers. The data revealed that the teachers' high CK performance in mental calculations with rational numbers is positively influenced by the existence of an extensive strategic repertoire. Furthermore, it was found that a high CK performance and an extensive strategic repertoire in mental calculations with rational numbers positively influence the PCK of mental calculations with rational numbers.