Dynamic properties of an existing nine-story reinforced concrete building were monitored during its demolition period, in Istanbul. Based on the municipal regulations, the number of the story of the building was reduced to five first by the removal of the top stories one by one. Later on, the remaining building was demolished by an excavator-like vehicle working on the ground. At the particular stages of the demolition, vibration measurements were taken from the present phase of the building to obtain its dynamic properties within the basis of operational modal analysis. The stage of the building and its dynamic properties were elaborated in detail to analyze the effects of the structural changes into the dynamic properties of existing reinforced concrete buildings. Thereby, the effects of floor removal and area reduction in the dynamic properties of existing reinforced concrete buildings were revealed.