Diaminoglyoxime (DAG) was prepared by adding glyoxal and hydroxylamine hydrochloride into a cooled solution of sodium hydroxide in water, and the single crystal of DAG was cultured by dehydration at high pressure. The crystal was characterized by using X－ray single crystal diffraction，elemental analysis and FT－IR measurement. The crystal is monoclinic, space group P2₁ with cell parameters: a= 0.6763(8) nm, b= 0.3578(4) nm, c= 0.9658(12) nm,β= 90.78(2)°, V= 0.2338(5) nm³，Z=2, D_c=1.678 g·cm^-3, F（000）= 124,μ(Mo Kα)= 0.146 mm^-1, final R indices ［I>2σ(I)］ R₁ = 0.0403, wR₂ = 0.1098； R indices (all data) R₁ = 0.0422, wR₂ = 0.1122. The obtained results indicate that DAG is a kind of planar symmetric molecules, and its crystal is acicular crystal. The thermal decomposition process of DAG was predicted by TG－DTG and DSC techniques with a heating rate of 10 ℃·min^-1. The measure results show that DAG melting at 203.5 ℃, has two steps of intense thermolysis between 209-212 ℃ and 212-240 ℃, and decompose completely up to 240 ℃.