The Touchahi earthquake of Aug 27, 2010 (M_N 5.9; IRSC- Mw 5.7; USGS) occurred at 19:23:49 UTC (23:53:49 local time on 5 Shahrivar 1389) in south of Damghan city. No foreshock were reported before this earthquake whereas 85 aftershocks (M_N 1-5) were registered by IRSC until 1 month after the mainshock. According to our field study after the event, surface rupture of causative fault was not observed but we measured some fractures related to this event with dominant strike of N120º-140º. According to our observations of 32 towns and villages that were damaged in this seismic event, maximum intensity (I₀) of VIII+ in MMI scale occurred near the Touchahi village in ~85 km south of Damghan city. Unfortunately in this earthquake 4 people were killed. Focal mechanisms of the Touchahi seismic event and its greatest aftershock is solved using the first P motion method. The fault plane solution show near vertical plane for the causative fault of the earthquake and suggests a left- lateral mechnism. The mechanisms associated with the fault show mainly left-lateral strike–slip motion, on a NE –SW striking fault plane. Based on location of the earthquake epicenter, its aftershocks location, the fault plane solution (left-lateral strike-slip with N039º strike and dip direction toward NW) and field observations, the causative fault of Touchahi earthquake is one of the active fault branches that is situated in north of Darestan mountain and south of  Touchahi, Koohzar and Kooshahi villages. This fault with left-lateral strike-slip mechanism by general strike of NE-SW and dip direction toward NW is indicated as Touchahi fault.