Abstract: In this paper, the conformation of the 5-substituted isopropylidene malonates was analyzed in detail according to the principle of the structural adaptability for the interreaction between the groups of molecules. The results agreed with their nuclear magnetic resonance spectra.(1) It was evident that the isopropylidene malonate had two energetically favoured chair conformations which could rapidly interconvert one another,(2) If the 5-substituted group contained aromatic group, such as phe-nyl, furfyl, the favoured conformation was that the aromatic ring tended to be parallel with the plane containing the two ester-groups. Here the boat conformation might be favourable. Therefore, the long range shielding effect of the aromatic ring toward the 2-methyl group could be shown. It means that the chemical shifts would move to higher magnetic field.(3) If the 5-substituted group was a carbonylmethyl group, the oxygen atom in carbonyl group approached to the three hydrogen atoms of the 2-methyl group. At that time a long range deshielding effect of the carbonyl group appeared.