Conformational analysis of 3-substituted thiophene carbonyl derivatives, thiophene-3- carbaldehyde and 3-methoxy carbonyl thiophene has been studied by means of NMR coupling constants, carbonyl shielding effects and solvent shifts, showing that S-cis form is preferable to S-trans. Moreover, the result of a semi-empirical calculation (CNDO/2 method) suggests that the most stable conformation may be the twisted form of S-cis type even though it may not be perpendicular one. This conclusion seems to be consistent with our present experimental results and other information through IR and ultraviolet spectroscopy.