We studied the most recent earthquake in Tehran occurred on 20/12/2017, at 19:57 local time, with aftershocks recorded by the local network of the Institute of Geophysics, University of Tehran and the International Institute of Seismology. We performed a fully nonlinear probabilistic relocation to get the best results of absolute locations. To increase the accuracy of earthquake locations, a one-dimensional velocity model was also calculated for this area. From the location point of view, the results indicate two seismic clusters, each showing the trend of north-east to south-west. Two depth profiles also show earthquake distribution starting from a depth of 10 and continued to 30 km with a dip of more than 70 degrees to the north. For further investigation, the main earthquake mechanism was studied by solving the focal mechanism using waveform modeling method. The mechanism obtained between the Eshtehard fault and the northern Ray with the east-west trends and the left-handed right-angle mechanisms. If we attribute the mechanism to the Eshtehad fault, we must consider the east- west plan as the responsible fault plan. However, due to the location of the earthquake and good station coverage and low error of location results, it can be said that the end of the Eshtehard fault is buried and has a flexure in the direction of approximately north of the south. Therefore, this mechanism represents wisdom of the hidden fault with the right mechanism.