High quality Fe3O4 thin film was synthesized by the magnetron sputtering method using Ar-H-2 gas. Giant magneto resistance of the trilayer using Fe3O4 was investigated, where the observed MR ratio was less than 1%. A phenomenological calculation was made to make the origin of the small MR ratio clear, using a parallel current model assuming five passes. It is concluded that the non-magnetic layer surrounded by two magnetic layers is necessary to have the same order resistivity and similar thickness with Fe3O4 layer in order to obtain the large MR ratio for the trilayer using FC3O4.