The role of molecular weight dispersion (Mw/Mn) of base phenolic resin was investigated in chemically amplified positive KrF resists. The resists consisted of tert-butoxycarbonyl (t-BOC) blocked polystyrene as base resin and 2,4-dimethylbenzenesulfonic acid derivative as photoacid generator, and the Mw/Mn value was changed as 1.2, 4.0, and 9.0. Not only dissolution rate contrast, but also the slope rn of log(dissolution rate)-log(exposure dose) plots increased with decreasing Mw/Mn. In the case of low Mw/Mn, the molecular structure becomes more homogeneous, so resist film could be dissolved uniformly via constant developer penetration into film. This uniform dissolution characteristics may be attributable to the origin of dispersion effects mentioned above. Furthermore, higher acid diffusion property and smooth pattern side wall were obtained without side wall roughness in lower Mw/Mn. This fact indicates that acid diffusion length is also uniform within resist film in the case of low Mw/Mn. Based on the experimental analysis, the clear relationship among Mw/Mn, dissolution characteristics and acid diffusion behavior in resist film was obtained. Moreover, it was found that lower Mw/Mn has advantage for improving inherent resist performance.