The attenuation of seismic waves with respect to distance is one of the most important parameters in seismological studies and earthquake engineering in each region. The purpose of this study is to estimate the quality factor of Shear waves (QS) how to absorb waves from the earthquake in the Central Alborz enclosed at 34º to 37º north latitude and 50º to 56º east longitude, using the spectral drop. For this purpose, the data from an earthquake of May 28, 2004, Firouzabad-Kojur and its aftershocks, which recorded by seismic networks of Sari and Semnan, affiliated with the Tehran University's Geophysics Institute used; In this study, QS for N-S and E-W components at the seven band frequency was estimated. The mean values of QS for two components obtained a relation of QS=73.54f 0.83 which show an increase in QS with increasing frequency. Q0 value for this area has an adjustment with seismotectonic of the region