Disclosed is an apparatus and method for forming an oxidation layer on a manifold block for a fuel cell stack, which forms an oxidation layer uniformly over the entire surface of a long and complicated internal flow field of a manifold block. In particular, the apparatus for forming an oxidation layer on a manifold block for a fuel cell stack includes: an electrolyte bath which contains an electrolyte required for the formation of the oxidation layer, an electrode for supplying a required electron flow for the formation of the oxidation layer from a power supply to the manifold block immersed in the electrolyte of the electrolyte bath and to the electrolyte, and an air supply for supplying oxygen to the electrolyte. Even more particularly, the electrode connected to the electrolyte is inserted into each internal flow field of the manifold block to provide an effective electron flow therein.