The focal mechanisms of earthquakes describe the fault plane orientation, slip vector and stress distribution in the region which can be exploited for studying geodynamic, seismotectonic, and seismic hazard analysis. In this study, to test whether S/P-wave amplitude ratio can improve the focal mechanisms of small earthquakes, a large earthquake has been selected so it can be possible to compute the focal mechanism using moment tensor inversion. The result for a large earthquake can be extended to small earthquakes. The focal mechanism of the first main event of the Varzaghan-Ahar earthquake occurred on 11 August 2012 has been calculated using three methods including, moment tensor inversion, P-wave polarity and S/P amplitude ratio with P-wave polarity. The uncertainty of the moment tensor solution demonstrates that the solution is reliable. The results demonstrate that two solutions including the S/P amplitude ratio with P-wave polarity and moment tensor inversion have rather same outcomes.