The initiation of plasticity and the subsequent state in the vicinity of a single grain boundary during indentation-induced deformation were investigated to understand an elementary step of a stress-strain behavior of polycrystalline materials. Nanoindentation measurements on several points on a single grain boundary and the grain interior of an interstitial-free steel and an analysis on the pop-in behavior and the plastic nanohardness were carried out. The pop-in load P-c that was obtained on the loading curve is different for each measurement. However, the loading curves overlap one another and the unloading curves coincide as well after the pop-in event. The nanohardness Hn has no dependence on the P-c in the range of 150-550 mu N. The relation between P-c and Delta h can be expressed as a simple cubic polynomial function based on a geometrically necessary dislocation loop model. The fitted function differed for various grains with different crystallographic orientations.