The proofs of God's existence are among the main questions of philosophers. This question has a special importance in Spinoza's works, because it constitutes the basis of his philosophy. In his main work, Ethics, he initiates the discussion with the definition of God and ends it up with man's noblest happiness, namely love of God. He deals with the issue in his different works and proved God's existence in two ways: a priori and a posteriori. He develops a new concept of God and considers Him as Unique Substance and Nature which contains everything and is consisting of infinity of attributes, including extension and thought. Spinoza's different proofs are similar to those of Descartes. Although Spinoza develops a new conception of God's existence, his proofs are in line with those of Descartes. In this paper, the writer attempts to explore different versions of a priori and a posteriori proofs in Spinoza's works and compare them with those represented by Descartes.