One of the methods for the determination of focal mechanisms of earthquakes is the linear inversion of moment tensor in time domain. That method is used here for determining of the focal mechanisms the Darbe Astaneh Silakhor earthquake of 31 March 2006 and two of its foreshocks and six aftershocks with local magnitude more than 4. The data of six broadband stations of the International Institute of Earthquake Engineering (IIEES) in epicentral distances less than 375 km are used. The focal mechanism obtained for the main shock is strike slip right lateral. The location of the main shock and the aftershocks and focal mechanisms show the Doroud fault as the cause of the earthquake. The variations of focal mechanisms of six aftershocks primarily indicate the local change in faulting and secondly the location of the aftershock with mainly normal mechanism can suggest the activity of the Qaleh Hatam fault. In this paper the main focus is on the main shock. Its centroid is obtained and the effect of frequency range in focal mechanism is studied by the inversion in nine small frequency bands and the optimum focal mechanism is found. Rupture length is also calculated. The uncertainty analysis confirms the acquired solutions.