The objective of this dissertation was to analyze three demonstrations of the existence of God in the Metaphysical Meditations of René Descartes. It contains three chapters. In the first one, we deal with general form of the method of Descartes, having the intention to show that this method will be applied to the Metaphysical Meditations influencing significantly on the tests of the existence of God. In the second chapter, where we are already talking about the Metaphysical Meditations, we withhold them in the analysis of the methodical doubt and cogitation. These two subjects are the basic antecedents for the tests of the existence of God, while through the doubt the resource to the knowledge of material beings and to the proper Nature is forbidden, and through the cogitation is discovered the only possible way to reach the God. In the third and last chapter, we present the first one, the second and third test of the existence of God. In all these three tests the main question was the application of beginning of specified cause to the form of the efficient cause. In the first test, God is inferred as the efficient cause of its present idea in the human intellect. In second, God is proven as a cause of the proper intellect that has its idea. Finally, in the last test, God is conceived as a formal and efficient cause of itself.