The concept of the"innate purity of the mind" (cittaprakrtiprabhasvara) is a very important notion in the Mahayana Buddhism and has a great influence on the Chinese Buddhism. It is often used synonymously with "tathagatagarbha" in many Mahayana texts. However, the origin of this concept can be traced back to the Agamas and the Nikayas. It is quite interesting to find such a concept in these sutras which vey much emphasize the concepts of impermanence and non-ego. The purpose of this study is to clarify the true meaning of this concept of the innately pure mind in the Agamas and the Nikayas. This paper is divided into three parts. In the first part, the original statements the entire collection of the Buddha's discourses. Since the formula of this concept involves the defilements of the mind, the second part of this study focuses on the types of the defilements which contaminate the mind. The analogies found in the texts discussing the defilements also help to further clarify the meaning and functions of the concept discussed in part one. As there is a lot of debate on this concept in the Abhidharma literature, the third part presents some of the debate to illuminate the issues concerning whether the mind can be truly polluted and the relationship between the mind and the defilements. In the conclusion, in addition to summing up the discussion on the true meaning of the concept of the innately pure mind in the Agamas and the Nikayas., the difference between its use in these sutras and that in the Tathagatagarbha sutras is also briefly mentioned.