The topic of this paper is Descartes' rationalist metaphysics. The aim is to present the most important elements of Descartes' metaphysics, with special emphasis on the novelties he introduces into this branch of philosophy. The method of philosophical analysis of the most important works of Descartes was used. The work begins with Descartes’ biography, in which one can see how three dreams made a turnaround in his life and influenced his work. To begin with, he introduced a new method into philosophy, starting from doubt and rejecting everything that can be doubted in the least. Then comes the undoubted truth, which he expresses with the statement of Cogito, ergo sum, and on which he will begin to build his philosophy. After realizing the certainty of his own existence, he brings evidence for God's existence, and then proves the existence of the material world, relying on God as the guarantor of true knowledge. In the end, he concludes that man consists of a protracted and thinking substance, failing to fully explain their relationship.