It has been widely known that photoresist (PR) will shrink after ion implantation, which contribute additional side effect on the PR edge placement error and PR effective thickness to against next implantation step, and such effect cannot be ignored with the device size shrinkage. KrF process is still the mainstream patterning solution for advanced logic implantation process. And there are two main category KrF resists: Low activation energy (Ea) Resist and High Ea Resist, which show different shrinkage behaviors, but the inside mechanism has not been discussed clearly so far. In this work, we studied the shrinkage behavior of two kinds of KrF resists (Low Ea Resist and High Ea Resist) under different exposure dose and under different implantation condition (acceleration voltage, dosage, and species). Resist shrinkage percentage and its dependence on time at room temperature are the focus of this study since multi-step implantation is widely used for device tuning purpose. It is observed that high Ea resist shrinks instantly after implantation and change little with time, while low Ea resist not only shrinks at the beginning but also shrinks continually with time at room temperature. The low Ea resist irradiated with ion implantation may undergo a two-step process: 1) polymer crosslinking and chain scission by bombardment of high energy ions; 2) deprotection by acid generated during implantation. We compared the shrinkage behavior with just exposure with different energy and just implantation with different voltage/dosage/species and observed some similar shrinkage trend, which may indicate similar mechanisms, however there still exist different contributions.