The cis stereochemistry of 6-(4-methoxy-phenyl)-1,5,7a-triphenyl-tetrahydro-imidazo[1,5-b][1,2,4]oxadiazol-2-one was studied by use of a PM3 semi-empirical quantum mechanical model, and x-ray crystallographic analysis. It crystallizes in the monoclinic space group P2(1)/n with a = 10.812(l) Angstrom, b = 16.464(2) Angstrom, c = 13.379(1) Angstrom, a = 90.00degrees, beta = 98.39(l)degrees, gamma = 90.00degrees, V = 2356.07(4) Angstrom(3), Z = 4, D-calc = 1.3067 cm(-3), F(000)=976.41, and mu = 0.086 mm(-1). The structure was solved by direct methods and refined to R = 0.066 for 1257 independent reflections [I > 4\sigma(1)]. The results from x-ray diffraction were seen to be generally consistent with the results from previously reported spectroscopic investigations, beside theoretical calculations, except for conformations of five-membered fused heterocycles. Two inter- and intramolecular weak interactions in addition to carbon atoms (Cl and C3) with different chiralities were found in the structure. The conformational study was performed by randomly scanning the potential energy surface belonging to the title compound with respect to selected torsion angles.