An environmentally favorable (chlorine-free) solid oxidizer, guanidinium dinitramide [GDN; NH2C(NH2)NH2N(NO2)2], was newly synthesized from guanidine carbonate [NH2C(NH)NH2·1/2H2CO3]. Two different crystalline forms (α-type and β-type) appeared according to the applied solvents and synthesis conditions. Moisture, during extraction, might become trapped in a crystal between inner molecules. Therefore, despite having the same chemical composition, Raman-IR and TGA-DSC revealed different physical characteristics of the two forms. Peaks of Raman shift near 1000cm−1 implied different chemical structures. Thermal analysis revealed an exothermic temperature 155.7°C for α-type but one of 191.6°C for β-type. The caloric value of α-type was 536.4J/g, which was 2.5 times larger than that of the β-type, which was 1310J/g. While the synthesized GDN of α-type showed a steep exothermic decomposition, the β-type was slowly decomposed after melting through an endothermic process. This work implied that despite of the same molecular formula some different core thermal properties would appear depending on synthesis conditions.