Molecular structure and conformational stability of E and Z geometrical isomers of propionaldehyde oxime (C(1)H3C(2)H2C(3)HNOH) have been studied by using the ab initio and DFT methods. The molecular geometries were optimized by employing the atomic basis sets 6-31G* at the HF-SCF and MP2 levels of theory in the ab initio method. The basis sets 6-31G and 6-31G* are used in the BLYP method of DFT to optimize the molecule. The optimized structural parameters of the above methods are discussed in the light of the electron diffraction results of the molecule. The variations in C=N bond length, C1C2C3 and C2C3M angles of the sp and ac conformers of E isomer and of the ap conformer of Z isomer have been discussed in terms of nonbonding interactions of CH3 and NOH groups. The rotational potential energy surfaces of E and Z isomers were obtained for the C2-C3 rotational angle of propionaldehyde oxime at HF/6-31G*, MP2/6-31G*, and BLYP/6-31G* levels of theory. The global minimum occurs at phi(CCCN) = 120 degrees and phi = 0 degrees for the ac and sp conformations of the E isomer and phi = 180 degrees for the Z isomer. The ac form is found to be more stable than the sp form by 0.16 kcal/mol in HF/6-31G* level of theory; this difference agrees very well with the experimental value of 0.15 kcal/mol. The rotational potential curve of Z form shows that it has large-amplitude motion. The chemical hardness values obtained for the different conformers of the two isomers are in disagreement with the statement that the higher stable conformation has higher chemical hardness, but the trend obeys the general trend of oxime molecules. The Fourier decompositions of the rotational potential of the propionaldehyde oxime are analyzed.