In this study, plasma sprayed (Mn,Co)3O4 (MCO) coatings with spinel structure are used to form protective layer for metallic interconnects of solid oxide fuel cells. The temperature during thermal spraying is high enough to change the crystal structure and conductivity of protective oxides. The structure change is caused by the removal of oxygen ions at higher temperature. This behavior is so called thermal reduction. Furthermore, the transformation of MCO spinel to a NaCl type structure causes the degradation in its conductivity by several orders of magnitude. With better understanding and control of thermal reduction and/or phase transformation, it is believed that the proper heat treatment is an adequate method to obtain desired properties for protective oxide. Thus, the objective of this study is (1) to investigate the influence of thermal reduction on the conductivity and crystal structure; and (2) to improve the performance of interconnect through proper annealing at desired temperatures. XRD, SEM, and impedance spectroscopy are used to characterize the protective oxides before and after annealing and thermal reduction.