Enthalpy of formation for three- and seven- membered heterocyclic theoethers (11 compound) in gas state have been determined by different theoretical methods: compo-site methods (Gn, CBS) and two levels of theory B3LYP/6-31g(d) and MP2/6-31g(d) via two approaches: atomization energy (AE) and bond separation (BS). Median abs-lute deviations (MAD) and standard deviations (SD) of calculation values from experi-ment data have been determined for every used theoretical method. It has been observed that the bond separation approach produce lower value of MAD in most of the theoretical methods in comparison to the atomization energy approach; where the G3MP2 method produce lower value of MAD, while the deviations corresponding to the two levels of theory MP2/6-31g(d)//B3LYP/6-31g(d) and MP2/6-31g(d)//B3LYP /6-31g(d) is close to the deviations of the composite methods. The 6-31g(d,p) basic set doesn't show substantial effect in the theoretical results.