Molecular parameters and lithographic performances evaluated with a 20 kV electron beam of a newly developed negative resist, chloromethylated poly‐2‐isopropenylnaphthalene (CM‐IPN), are presented. IPN polymers with narrow molecular weight distribution (MWD) were obtained by anionic polymerization. Subsequent chloromethylation can enhance sensitivity with slight broadening of MWD. Observed contrast (γ) values of ∼2 are almost independent of sensitivity. The reactivity evaluated asMw ⋅ D0(Mw, molecular weight;D0, gelation dose) is about 0.18. Linewidth patterns of 0.1 μm or less with an aspect ratio of 5 can be delineated. High dry‐etching durability and excellent thermal stability owing to aromatic rings and the high glass transition temperature (230 °C) are also confirmed.