Abstract: Mega-gear measurement has been an unsolved problem in the field of dimensional metrology.The critical point is how to determine the position relation between the gear measuring machine and mega-gear.An In-site Mega-gear Measuring System with a Laser Tracker was developed,which indicates a total solution to mega gear measurement.In this case,a laser tracker is employed to detect the positions of the measuring machine and mega gear.Two positioning methods were discussed for merging data generated from different parts of the in-site measuring system.Two positioning modules of the in-site mega gear measuring system with laser tracker were proposed to establish a new coordinate for the measuring machine under the coordinate of the laser tracker.Both of the so called "straight line vector method" and the "integrated mapping method" were analyzed,the difference and the area of application were also discussed.The correctness of them is proved by simulations: both of the theoretical error and the computing error can be ignored in this measuring system,without taking the system error into account.