The single crystal of cobalt compound of 3-nitro-1,2,4-triazol-5-one (NTO) ([Co(H2O)6](NTO)2•2H2O) was prepared and the crystal structure has been determined by a four-circle X-ray diffractometer. The compound is monoclinic with space group Cc and unit cell parameters of a=2.3163 (2)nm, b=0.6451 (1) nm, c=1.4123 (1)nm, β=123.96 (1)◦, V=1.7504 (3)nm3 and Z=4. Based on the thermal analysis, the thermal decomposition mechanism of [Co(H2O)6](NTO)2•2H2O has been derived. From measurements of the enthalpy of solution in water at 298.15 K, the standard enthalpy of formation, lattice enthalpy and lattice energy have been determined as −(2594.7±4.5), −2753.19, and −2718.19 kJ mol−1, respectively.