A new MOF of [Co(HCOO)2] · 0.33DMF (I) was prepared and characterized by IR, powder X-ray diffraction, and single-crystal X-ray diffraction (CIF file CCDC no. 1571166). Complex I exhibits a diamond framework based on Co-centered CoCo4 tetrahedral nodes, in which all metal ions have octahedral coordination geometry and all HCOO groups link the metal ions in syn-syn/anti modes. The magnetic property shows a main antiferromagnetic interaction and the ferromagnetic phase transition below 2 K. Furthermore, the photocatalytic properties of I for degradation of the methyl violet and Rhodamine B have been explored. [ABSTRACT FROM AUTHOR]