The preparation and characterization of [Co I I - H S (dpa)(3,5-DBSQ) 2 ] are presented first time. From the magnetic curve, the phase transition temperatures, T c is 380K. To our knowledge, the T c value is the highest among the species exhibiting thermal valence tautomerism. After illumination with 532nm light at 5K, the magnetization values increase from μ e f f =1.73 to 2.0μ B , showing valence tautomerism. The metastable state will recover to the original state with the increase of temperature. The alternate illumination with 532 and 830nm light at 5K can induce a reversible change in magnetization. The quick response to light means that this compound can be used for the development of optical-magnetic switch.