Mental arithmetic has recently gained a higher profile in primary school mathematics. The study aims to reflect didactical background of mental arithmetic in number and operations curriculum for primary school mathematics. In order to attain these purposes, the present paper describes the meaning of mathematical literacy and didactical background of mental arithmetic on which have been laid emphasis in relation to mathematical literacy in many countries. Also it shows current suggestions for mental arithmetic instruction in Everyday Mathematics Project in USA, Numeracy Number Project in Great Britain, TAL project based on Realistic Mathematics Education in the Netherlands, and mathe 2000 project in German in order to gain practical ideas for teaching mental arithmetic. Furthermore, it discusses mental strategies of students and didactical models for improving mental arithmetic instruction based on the results of many researches. Under these theoretical foundations, it is analyzed how mental arithmetic is developed in our number and operations curriculum, focused on mental strategies and didactical models. Finally, implications for improving our mental arithmetic instruction are discussed.