The topography of water-soluble polymer chains immobilized on a rigid polymer surface underwater was studied using atomic force microscopy(AFM). As the substrate polymer a smooth poly(ethylene terephthalate) (PET) film was employed and polymer chains of various lengths were chemically immobilized by surface graft polymerization of 2-(dimethylamino)ethyl methacrylate onto the film using the W-induced graft polymerization method. The degree of polymerization (N) of graft chains ranged from 28 to 960. Except for the shortest chains (N = 28) the graft polymer chains were found to stretch out on the substrate underwater, forming a brush structure. The graft layers exhibited inhomogeneous structure, that is, clusters. This cluster formation was ascribed to substantially poor solvency of water for the graft polymer chain. AFM scanning performed at varying loads from 0.25 to 10 nN underwater revealed that, with the increasing graR chain length, AFM images underwater exhibited deformation (tilting) of the clusters at loads higher than 2.5 nN, unlike those in air. when a fixed area of the grafted surface was subjected to cyclic scanning, the tilting direction of clusters was completely in accordance with the scanning direction, and the image of tilting clusters was reproducible. This suggests that the tilting of clusters is reversible, recovering to the previous unperturbed shape as soon as the loading tip has been taken away. The lateral structure of the polymer chains grafted onto the smooth surface underwater was compared with that predicted by brush theories.