The standard molar enthalpy of formation of trimethylene urethane (1,3-oxazinan-2-one) in the crystalline state at 298.15 K was derived using combustion calorimetry. Standard molar enthalpies of sublimation and vaporization of this compound at 298.15 K were derived by the transpiration method. The enthalpies of fusion were measured using differential scanning calorimetry technique. Thermodynamic data on trimethylene urethane available in the literature were collected, evaluated, and combined with our experimental results. Quantum-chemical calculations of the molar enthalpies of formation of the compound in the gaseous state were performed using the G4 method, and the results are in excellent agreement with the recommended experimental data. Thermodynamic properties of trimethylene urethane in the ideal gas state were calculated from molecular and spectral data in the temperature range from 298.15 to 1500 K. The strain energy values (E-str) of different six-member cyclic compounds were estimated.