One of the most important parameters necessary in the seismic force determination is the building’s fundamental vibration period, which is also useful in “health monitoring” procedure for the existent buildings, during their exploitation. The fundamental vibration period can be determined by theoretical, empirical or experimental methods. In particular, the experimental method it is known as AVT (Ambiental Vibrations Tests) procedure, which intensively studied today. In this paper it was performed a comparative analysis between fundamental vibration periods values, determined for an existent building, by using a numerical method (finite elements program - ETABS), some direct methods (empirical formulas), and an experimental method (dynamic recordings, based on AVT), using special devices as accelerometers from KINEMETRICS brad. The existent building has “house flats” destination, and it is historic monument, located in Bucharest city, which it was built in ~1930 period, and it has basement and 4 floors. The building’s structure is done half by masonry walls with concrete slabs and half by RC frames (columns and beams), with concrete slabs. The main goal of the analysis is to emphasize the advantages and disadvantages of each studied method.