Dinitrourea is an important precursor in the synthesis of the cyclic nitroamine energetic compounds. Dinitrourea was synthesized through nitrification of urea by 100% HNO₃/20% oleum via mononitrourea as an intermediate. Under the optimal condition: 100% HNO₃/20% oleum(V/V)=0.6/1,temperature of the first stage at -15 ℃ and temperature of the second stage at 5 ℃,reaction time 50 min,the yield of dinitrourea is 83.2%. The structure of dinitrourea was also estimated by a B3LYP method based on 6-31+G (d,p) basis set and the stable geometric configuration was obtained. The thermodynamic properties at different temperatures were obtained from vibrational analysis,and the relationship between temperature and thermodynamic properties was deducted. Results show that dinitrourea has two stable conformations with energy difference of 12.621 kJ·mol_-1. The calculated vibrational frequencies and intensities of DNU are in good agreement with the experimental values, and the values of E^Θ_t,m,C^Θ_p,m and S^Θ_m are increased with increasing of the temperature.