Using amplitude spectra and polarity (ASPO) is a way for determining focal mechanisms of earthquakes. In the paper, the method has been used to determine the focal mechanism of Zarand Dahuiye earthquake dated 22 February 2005. The method uses frequency-wave number approach for calculating Green functions and inverts full waveforms (from P arrival time up to the end of surface waves). At first strike, dip and rake is determined by grid search for different depths, then the optimum solution is chosen according to the minimum of the error values. At the end, correct rake angle is determined by at least one polarity. The input seismograms are from broad band stations of the International Institute of Earthquake Engineering and Seismology (IIEES). The epicentral distances of stations used are from about 91 to 459 km and applied frequency band pass is 0.01-0.08 Hz. In this study the best fit is resulted at depths from 4 to 5 km and the optimum focal mechanism found is 226, 38 and 106, respectively for strike, dip and rake. The results are compared with independent focal mechanism determination for the earthquake.