In apartment buildings, floor-impact sound has bean regarded as the major source which induces complaints from residents. It is mainly due to the use of light-weight structures. The vibration produced by impact on one part of an apartment building would travel as far as the other parts of structure with a little alleviation. As a result, the impact sound from upstairs has been regarded as a main source of noise causing discontentment among occupants. This study was carried out to measure the floor-impact sound levels and evaluate the insulation performance of floor-impact sound for nine apartment buildings. The floor-impact sound levels were measured for twenty-five On-dol floor structures and various factors which influence the floor-impact sound were analyzed.