The structure,enthalpy of formation,density and thermal decompostion mechanism of N-methyl-N'-methoxydiazene-N-oxide(MMDO) were studied based on density fuctional theory,and compared with its nitramino-isomer dimethylnitramine(DMNA). Results show that Z-conformation of MMDO is more stable than that of E-conformation,and Z-conformation of MMDO is the main existence form. The energy barriers of isomerizations between Z and E forms are higher than that of thermal decomposition,accordingly the isomerizations do not occur in the conditions of thermal decomposition. The enthalpy of formation (42.50 kJ·mol^-1) and thermal stablily of Z-MMDO are higher than that of its nitramino-isomer DMNA(-12.74 kJ·mol^-1). However the density(1.331 g·cm^-3) of Z-MMDO is slightly lower than that of DMNA(1.363 g·cm^-3). CH₃O—N and CH₃—O dissociations are two competitive thermal decomposition ways of MMDO.