The single crystal of lead salt of 3-nitro- 1, 2, 4-triazol-5-one (NTO), [Pb(NTO)2.H20] was prepared and its structure was determined by a four-circle X-ray diffractometer. The crystal is monoclinic, its space group is P21/n with crystal parameters of a=0.7262(1) rim, b=1.2129(2)nm, c=1.2268(3)nm, [3=90.38(2)o, V=1.0806(2)nm3, Z=4, Dc=2.97gcm-3, p.=157.83cm-1, F(000)=888. The final R is 0.027. By using SCF-PM3-MO method we obtained optimized geometry for [Pb(NTO)2.H20] and particularly positions for hydrogen atoms. Through the analyses of MO levels and bond orders it is found that Pb atom bond to ligands mainly with its 6p, and 6py AOs. The thermal decomposition experiments are elucidated when [Pb(NTO)z.H20] is heated, ligand water is dissociated first and NO2 group has priority of leaving. Based on the thermal analysis, the thermal decomposition mechanism of [Pb(NTO)2.H20] has been derived. The lattice enthalpy and its lattice energy were also estimated.