The decomposition mechanism of hydrazine nitrate in nitric acid solutions was investigated using quantum chemistry calculations combined with the polarizable continuum model at the CBS-QB3//ωB97X-D/SMD level of theory. These calculations provided a detailed kinetic model incorporating rate coefficients and thermodynamic data. Rate coefficients were determined using traditional transition state theory, while diffusion-limited reactions were modeled based on the Einstein-Stokes equations. The resulting model comprised the kinetics for 108 reactions and thermodynamic data for 58 species. This model was validated by comparing simulations of the variations in chemical species during the decomposition process to experimental data acquired under isothermal conditions at 100 °C. The model was found to accurately reproduce the concentration changes of N2H4, HN3, and NH3 and also explained the reaction mechanism. The thermal decomposition was found to proceed via two parallel paths: N2H4 + HNO3 → H2O + HONO + N2H2 and N2H4 + HONO → HN3 + 2H2O. Following these reactions, a portion of the HN3 decomposes to produce NH3 through a multistep process. A sensitivity analysis showed that the rate of decomposition is greatly affected by the pH of the solution.