Using the moment tensor inversion in the time and frequency domain we calculated the focal mechanism for 30 Alborz earthquakes with Mw greater than 4 occurred between 2000 to 2020. The focal mechanism results show that most earthquakes near or over Khazar Fault have a thrust focal mechanism dipping southward while on its western end where connected to left-lateral strike-slip Rudbar Fault using Lahijan Fault, it changes to left-lateral strike-slip. Our results show that the mechanism of Khazar Fault at depth is different from surface observations. Focal mechanism of earthquakes in the central desert of Iran and in the southern wedge of Central and Eastern Alborz indicates the change of stress regime from tension to compression. Our study for the Miami Fault, where its mechanism was not obvious before, suggests an almost pure left-lateral strike-slip mechanism. Earthquake mechanism in Shahroud Fault system from Firoozkooh to Astaneh in Central and Eastern Alborz was almost observed to be left-lateral strike-slip, while the mechanisms of earthquakes located near this fault system are often thrust, The consistency of maximum stress direction of the two fault systems implies that both fault systems have emerged in a similar regime of regional stress. The mechanism of earthquakes at the eastern end of the North Alborz fault varies from dominant strike-slip to dominant thrust. It seems that earthquakes have occurred on the main direction of the North Alborz fault have a strike-slip mechanism while the earthquakes that have occurred on sub-directions perpendicular to the main direction of stress have a dominant thrust mechanism.