The present work aims to study the thermal stability and thermodynamics in the process of synthesizing 1, 3-dimethylurea. The HPLC spectrograms were used to study the impacts of reaction time and temperature on the thermal behaviour of the ammonolysis reaction system. Rate equation of the ammonolysis reaction can be summed up as c(t) = 1016.24*e-(t/47.9403) +16.1935. Thermal stability curve also can be determined. Experimental results revealed that the process of synthesizing 1, 3-dimethylurea was of pseudo-first-order reaction kinetics and k value was 0.0163 min-1. Thermodynamics results indicated that enthalpy was -1.92 kJ/mol, Gibbs free energy was -8.51 J/mol, and entropy was 15.96 J/mol*K. The reaction was an exothermic, spontaneous and entropy increasing process. Based on thermal stability and thermodynamics study, synthesis of 1, 3-dimethylurea was unfavourable at higher temperature as well as long reaction time.