Mathematics education starts from learning the concept of number. How the children at the beginning of school age learn the concept of natural number is therefore important for their future mathematics education. Since ancient Greek period, the concept of natural number has reflected various mathematical-philosophical points of view at each period and has been discussed ceaselessly. The concept of natural number is hard to define. Since 19th century, it has also been widely discussed in psychology and education on how to teach the concept of natural number to the children at the beginning of school age. Most of the works, however, were focused on limited aspects of natural number concept. This study aims to show the best way to teach the children at the beginning of school age the various aspects of natural number concept based on activistic perspective, which played a crucial role in modern mathematics education. With this purpose, I investigated the theory of the activistic construction of knowledge and the construction of natural number concept through activity, and activistic approaches about instruction in natural number concept made by Kant, Dewey, Piaget, Davydov and Freudenthal. In addition, I also discussed various aspects of natural number concept in historical and mathematical-philosophical points of view. Based on this investigation, I tried to find out existing problems in instructing natural number to primary school children in the 7th National Curriculum and aimed to provide a new solution to improve present problems based on activistic approaches. And based on activistic perspective, I conducted an experiment using Cuisenaire colour rods and showed that even the children at the beginning of school age can acquire the various aspects of natural number concept efficiently. To sum up, in this thesis, I analyzed epistemological background on activistic construction of natural number concept and presented activistic approach method to teach various aspects of natural number concept to the children at the beginning of school age based on activism.