In this study, we collected 55907 Pg arrival times from 3963 (re-)located earthquakes recorded by 353 seismic stations. We have developed a linear non-iterative damped least square method to get the 3D velocity model in NW Iran. Our result indicates that the depth of sedimentary layer in Zagros varies between 2 and 8 km beneath the Lurestan arc. We have found an unusual thick (~12 km) low velocity zone in Alborz which exists only near the Rudbar valley. Our velocity map shows that the average basement depth in NW Iran is about 12 km. At depth of about 17 km we have found some low velocity zones near the Alborz region crossing the basement perpendicular to the mountain belt strike. In Zagros, the Pg velocity is low at 17 km depth which is limited to MFF from its western part.