Guanidine dinitramide (［(NH₂)₂CNH₂］+N(NO₂)₂-, GDN) was prepared by mixing ammonium dinitramide (ADN) and guanidine hydrochloride in water, and its structure was firstly determined by singlecrystal Xray diffraction. The crystal is triclinic, space group P1 with crystal parameters of a=0.8332(5) nm, b=0.9306(6) nm, c=0.9878(6) nm, α=84.659(11)°, β=69.213(12)°, γ=67.451(12)°, V=0.6605(7) nm³, Z=4, μ＝0.159 mm^-1, F(000)=344, and D_calc=1.671 g·cm^-3. The thermal behavior of GDN was studied with DSC and TG/DTG methods, and its thermal decomposition process can be divided into four stages, and the third stage is an intense exothermic decomposition process. The apparent activation energy and preexponential constant of the exothermic decomposition reaction are 118.75 kJ·mol^-1 and 10^10.86 s^-1, respectively. The critical temperature of thermal explosion is 164.09 ℃. GDN presents higher thermal stability than ammonium dinitramide (ADN).