To directly characterize the thermoresponsive structural changes of a poly(N-isopropylacrylamide) (PNIPAAm) graft layer at the microscopic level, the force-distance curve (f-d curve) was measured on a well-tailored end-grafted PNIPAAm surface in aqueous solution at 25 and 40 degreesC, using an atomic force microscope (AFM). The PNIPAAm surface was prepared by an iniferter-based photograft polymerization technique. The approach trace of the f-d curve exhibited a steric repulsion profile at 25 degreesC, while the range of repulsion decreased 1/10 to 1/20 at 40 degreesC, confirming the ascending-heat induced collapse of the PNIPAAm graft layer. The change in thickness of the graft layer was complementarily measured from the scanning images of the boundary between the grafted and nongrafted regions under well-defined scanning forces. The thermoresponsive characteristics of the PNIPAAm graft layer including its interaction with proteins and the applied-load dependence of the measured graft thickness are discussed.