This paper aims to present Plato's theory of ideas. Plato was the first of the systemic philosophical writers in the West to start thinking about writing a wide range of topics that are still relevant today. The presentation of the theory of ideas in this paper begins through the interpretation of its appearance in the dialogues in Fedon and The State, and then through the critiques of the theory of ideas as found in the relevant historians of ancient philosophy. For Plato, ideas are eternal and unchangeable, and sentient beings are changeable and imperfect, and exist only by participating in ideas. Plato wrote all his presentations in the form of a dialogue that is not only a conversation between the participants, but also a way of coming to the truth. Plato's dialogues play a very important role in interpretation of his theory of ideas. Through them we approach Plato's way of thinking and seeing the world. Plato's dialogue Fedon is one of the most remarkable achievements of the human spirit where the notion of idea plays an important role. One of the important facts stated in Fedon is that we cannot know ideas by using any of the senses, but by pure thought. The State is also one of Plato’s fundamental works, representing Plato’s thinking as an idealistic utopian. The theme of the state is a good idea, and the state is used for methodological reasons as an enlarged individual. Finally, when he speaks of the idea of good, Plato argues that it is necessary for justice and other virtues because they can be fully realized only in the light of something greater than themselves.