A nail gun includes a fastener drive track capable of receiving fasteners having different length. A faster positioning assembly includes at least one swinging member and a stop portion. The swinging member is elastically and pivotably disposed at a side of the fastener drive track. The stop portion is formed at an end of the swinging member and received in the fastener drive track. The stop portion is configured for positioning fasteners prior to the fasteners are hit, and the stop portion retractably exits from the fastener drive track when a long fastener is received in the fastener drive track. The fastener positioning assembly enables the nail gun to drive fasteners having different length, as such, it is very convenient to use such nail gun to join different workpiece having different thickness on an object.