Transition-metal oxides have attracted much attention due to its abundant crystalline phases and intriguing physical properties.However,some of these compounds are difficult to be fabricated directly in film form due to the ease of valence variation of transition-metal elements.In this work,we reveal the reversible structural transition between SrVO3 and Sr2V2O7 films via thermal treatment in oxygen atmosphere or in vacuum.Based on this,Sr2V2O7 epitaxial films are successfully synthesized and studied.Property characterizations show that the semitransparent and metallic SrVO3 could reversibly switch into transparent and insulating Sr2V2O7,implying potential applications in controllable electronic and optical devices.