AbstractPositive photoresist systems comprising phenolic resin and quinonediazide are studied in detail. The structure of several novolak resins, which were synthesized from multi‐substituted phenols, are analyzed with13C‐NMR. It is proved that these phenols are useful for the structural control of novolak resins. Phenols with two methyl substituents at meta‐positions especially show a remarkable structural effect to control methylene bond types of novolak resins.The alkaline dissolution rate is also investigated on novolak resins synthesized from multi‐substituted phenols to understand both the dissolution inhibition effect and the dissolution promotion effect. Key‐structure of novolak resins for realizing high resolution is elucidated.In addition, the molecular design concept of novolak resin is clarified from several standpoints. Unsubstituted aromatic carbon of resin, hydrophobic effect of methyl substitutent and suitable structure of low Mwparts of novolak resin are also explored.Furthermore, it is proposed that a resist using a resin, which is based on the molecular design concept, has enough potential to produce critical dimensions of 0.2 μm or less taking advantage of phase‐shiftin