My paper is essentially concerned with Hans Reichenbach's work on the theory of relativity, Relativitätstheorie und Erkenntnis A Priori, published in Berlin in 1920 insofar as it shows clearly his intellectual transition from neo-Kantianism to neo-positivism. Nevertheless, First is very briefly recalled Cohen's main reflections on Einstein's theory of relativity within the limits of the special theory. Indeed, since Cohen died in 1918, he did not have the time to take into account the main teachings from the general theory of relativity. So, Cohen was convinced that his interpretation of Einstein's theory of relativity remained in accordance with Natorp who had written in 1910 that Einstein provides a confirmation of idealism: "Bestätigung des Idealismus".On the contrary, the rise of Einstein's theory of relativity gave to Reichenbach the opportunity to recast his own philosophical point of view. To give an account of Reichenbach's philosophical reception of Einstein theory of relativity, we will have to raise four questions: What were the links between Reichenbach and Cassirer? Which statements of the theory of relativity were in conflict with the Kantian a priori? What kind of discrepancies opposed Reichenbach to Kant? Is the recasting of apriorism a consequence of the theory of relativity?